I feel that your concerns are justified. I also think that, given the fact that your fiance sounds like a man who is not a jerk, you will find that he will be over the moon happy and excited to have a baby with YOU, his WIFE, the woman he LOVES.
Remember, he has not experienced that yet.

That said, it is entirely possible that you will feel an immediate difference between your love for your step son and your love for your biological child. That is NORMAL. Don't let it flip you out!

When you marry a man with kids, you will always have compromises. It will never be just the two of you. I don't think it's possible to be a stepmother and not have resentments.

I think (speaking from experience) if you can learn to let go of resentments somehow and recognize how negative they will influence you as a person if you do not, then you'll be ahead of others in your position.

Your world should not revolve around whether he adores your children with him the same or more than his existing son anymore than he spent time wondering if you would be able to accept his son. You just did it. I'm sure he did not obsess over it.

You brought no kids to the marriage. He's one lucky guy. Make sure you take care of yourself. Being a stepmother is a brutal job. Most women have no idea what they are getting themselves into when they marry men with children.

You said that you feel like you will never measure up. Please reconsider your thinking. The BM does have leverage, and we've all had to deal with other people (sometimes LOTS of other people) using leverage to manipulate our husbands. That is so hard to watch. But you are his safe spot, his safety net. You're his best friend. Don't underestimate yourself.

As for how your DH will love your child.. here's what I suspect might happen.

When you and he have children together, he is going to bond with them big time because for the first time in his life, he'll actually live full-time with his kids. The relationship he has with them will be difficult for your SS to accept. Your SS is bound to feel jealous, and with a lunatic mother training him that you and your DH dumped him, are evil and that he should just forget you, he might turn into one spiteful character.

If/when that happens, your SS is going to become more challenging for your DH to love. Your DH will likely feel guilt toward him and feel obligated to chase him, but the truth is, it will be harder for him to have a mutually loving relationship with him than it will be with your kids.

If your SS turns out like many stepchildren we've seen here, he will focus his anger and resentments on you. His mother will have helped a whole lot. He'll go from a sweet little kid, to a child who might be disrespectful toward you, snubs you and tells his daddy he doesn't want to be around you. At the heart of his issues may be pain, but it will come out in forms of spite. It is extremely difficult to continue caring about this sweet little kid, and to empathize with him, when he turns on you and tries to make your life hell.

If you can realize, when/if this happens, that it is very common among stepfamilies, than possibly you can skip past the step where you spend years wondering how you could possibly have been so horrible to his kid for him to hate you so much.

It's sad for him to be the product of divorce. The truth is, you weren't there when he was conceived and you had no part in the decision for him to exist. You can't stop him from feeling like his father remarried and made another family. Some of the stepmothers here have bent over backwards trying to be so kind and welcoming to their stepchildren, and it hasn't mattered at all.

For your SS's sake, I hope he can resist the urge to go to war with you and your DH. For your sake, I hope you can deflect his attempts if he does, and enjoy your family.

In several years, BM's leverage will become less. You will be the mother to your DH's fulltime kids. And yes, he will love them just as much the second they are born.

I hope that helps. Of course, I'm not a predictor of the future, just a regurgitating past experiences and stories from others.

It will also help you if you stay true to yourself....always, always, always. Don't take to heart the weird, sometimes hurtful comments that may be slung your way by your skid. When your kids begin to talk and walk, they make mistakes. You forgive and reteach the correct way to take steps and form words/sentences. Same thing with a skid.

The very first moment you detect something is changing with your skid, reteach him what you expect. And do NOT compromise what you believe in. Throughout the years, your skid may experience PAS from BM. Please do yourself a favor and ignore it. With the BM in my world, she tried and tried everything (and I mean everything) to split DH and I up. It didn't work. Things throughout the years have been some of the most painful things I have ever had to deal with. I never, EVER responded to her. But, BM did, eventually, fade out of our conversations. It took years, but it did happen.

My point is...since you're just beginning in this...do NOT let anything in this world change who you are.

You are giving her way too much power. How can someone else diminish the joy of having your own child? Your first child with your fiance is still your first child together. The excitement will be even better because you are in love and are a family. You will have that solid foundation that he and his ex did not have.

His ex did not TAKE anything from you, she did not get pregnant all by herself. Yes, she will always use the kid as leverage but she will only have as much power as you and your fiance give her. Don't let the petty things she says get to you, that's what she wants. It is WHY she says them. Understanding this helps to disregard them.

She is in your life, nothing you can do about that. Get her out of your head. I know it's not easy. Continue to be the best person you can be, to your fiance, to your stepchild, and to yourself. You certainly don't need to "measure up" to her. Limit your contact with her if that's what it takes.

Also, I think you need to deal with the resentment, as I think some of it might be misplaced. Is there any chance you also resent your fiance (even a little?) but don't realize it or want to admit it? He is the one who got her pregnant. Forgive him and let it go or it will definitely seep into other areas.

I do not have children and it has never bothered me that another woman has had a child with my husband. I know he loves me, and is with me because of that love, not because I had his child.

I can relate from the guy perspective. My wife had our son (my SS-20) when she was 16yo. We met when SS was 1yo and my DW has recently turned 18. We married shortly before SS turned 2yo. I am not his biodad but he is my son just as you indicate that your SS is your son. And ...... your son is YOUR son. You are the only REAL mother he has since no psycho manipulative POS BioParent can be a REAL parent. So take comfort in the FACT that YOU are his only REAL mom. Just as my SS is MY son and I am his onoly REAL dad. A manopulative worthless POS can no be a real parent. Your SS's BM and my SS's BioDad fit this rule.

Also, you will have your DFs first child ... with you. Your first child will be his first child with the two of you together. Firsts can come in many configurations.

As for BM screwing with your head ... that is your choice to let her. So, stop letting her screw with your head. My SS's SpermIdiot has never stopped pinging the kid with questions of "Does your mom still love me...", etc... She just laughs her ass off any time that shit comes up. Get in her head instead of letting her take up space in yours. Yuor are you SS's real mom so start presenting the facts of your blended family situstion so that BM clearly gets that message. Do nto react to her, proactively lead her where you want her to go. Keep being a good mom to YOUR son (SS) and keep leading BM around by her swine snout ring rather than letting her play with you.

The beauty of this model is that baring the idiot asses of the blended family opposition gets easier and easier over tome and it can be sooooo much fun. I made baring the SpermIdiot's and SpermGrandMa's asses a sport and I got very good at that sport. The facts of their behavior, the CO and any supplemental local rules and state laws are the best tools for baring their idiot asses.

Good luck and have fun.