Where is their mother and why isn't she helping them with their school work? Why isn't he helping them with their school work too?Why would he become resentful towards you considering they are not your kids? Btw it doesn't get better over time. It gets worse! Love yourself enough to be in a relationship where your love is not based on what you do for someone else's children and you are not playing free nanny in order just to have a man. It's not worth it. You deserve better. Love for someone else is not enough . Love yourself if you believe in love. Self love is the most important form of love. 

Dating a man with kids doesn't mean you become another parent. Hell, marrying a guy with kids doesn't mean you become another parent. He chose to have these kids, he can do their school work with them and entertain them. That is not your responsibility at all. If you want to help, fine - but he has no business expecting it of you.

Seems like a lot of men expect their GFs and wives to take over parenting for them, or at least help, but that's not necessarily part of the deal. You can stop hating your life by either breaking up with this guy, or negotiating a new deal where he's the parent and you get to live your life as a childless person.

If he doesn't want to be a parent, he shouldn't have 50/50 custody.

How do I stop hating my life when kids are here? 
Good question, you can plan things you've always wanted to do, or need to do, such as visit friends or family, get a mani/pedi, read a book, ride a bike, take a dance class. Your bf has kids; you do not. To be fair, maybe you should try to fit in one day a month or so to get to know them and for blending purposes BUT do not make them your responsibility or YOU will become resentful. 
 
Is this normal? Yes, this is a variation of some normals I see, where stepparent is afraid to make bf/ husband resent her for having a life apart from them. But overall, no this is not normal in that it shouldn't even be a norm. 

am I in the wrong situation? To be very brutally honest with you, it sounds like you might be UNLESS you are able to free yourself from all responsibility towards these kids AND your bf is accepting of it. Nothing you do is ever going to be good enough for those kids simply because you aren't their mother. So don't  force it and focus on you, your goals and your dreams. 

does it get better? Yes and no again. Yes it gets better in that they grow up so need you to do less for them, but no in that the older they get the bigger the problems turn. For example my SS20 since I met him at 12 has always had daddy issues and loves his mom in spite of her crazy self. It affects him in a negative way to where he shared with me recently that he was experimenting with drugs under our noses and get this: He feels I could have And should have helped him!!! never mind the fact that he already has 2 parents who weren't checking on him the way they should have, it was somehow my fault also! SS15 was 7 when I met him: he had trouble with authority, specifically women and 8 years later he still does. I'm afraid they will never move out of the house and H is ambivalent about it all. After all, those are his kids and I'm sure he doesn't really want them to move out, he only feels that way because of me. So no longer do I help with homework or feel like I have to cook or clean for them or keep them entertained but damn if I still want my freedom back.

I don't want to be involved in entertaining the kids, but I feel guilty if I leave their dad to do it all on his own and feel he will begin to resent me if I don't.

If you don't want to entertain his kids, don't. Why should you feel guilty that he has to tend to HIS kids on his own? Who would entertain them or help with homework if he didn't have you for a GF?

If he were to start resenting you for not helping with his kids, then he is with you for the wrong reasons. He should not expect you to entertain his kids or help with homework, they have two parents.

The more you do, the more he wille expect, and you will resent him. Which, you are already resenting the situation, itself.

Between this and your other post, you seem really concerned with your BF resenting you for your feelings, for wanting boundaries, and for wanting a certain amount of comfort level in your own home.

Yes, this might be in the wrong relationship for you.

Does he ask you to help/entertain the kids?  Or is he pushy?  Is he trying to form a family unit, but not considering all angles? Does he ask you to help with the 8 year olds school work? 

You said that you don't "want to be involved in entertaining the kids, but I feel guilty if I leave their dad to do it all on his own and feel he will begin to resent me if I don't.", but the big question is, has he made you feel guilty or shown signs of resentment?  If he has, then you have to be honest with him and say whatever your heart needs to say.  If he hasn't and that is your heart feeling guilty, the you should consider communicating that with him.  If you are doing things that you don't like, to keep a guy, then you aren't being honest with him, or yourself, and you both deserve better. 

I'm not going to flame on the guy without data.

Does he have any inkling about how your feel, or is this a situation that he has not clue about?  If he doesn't know, then you both deserve an honest conversation because he might be thinking that things are great and moving a direction more long-term, marriage, etc.

Situations like this suck, but if your communication is solid, then a solution should be in there somewhere.