I appreciate you feel like considering leaving and finishing your relationship is effectively allowing sd to "win" but in all honesty you cannot allow that aspect of your feelings to cloud whether this relationship is HEALTHY for YOU. Is it?

You mention that your DH believes his DD over you - regarding what exactly? I think that is a massive red flag and I sort of know what that feels like, as my own DH tried that one on me when it came to my ss. I blew a gasket and in all honesty if he had not acknowledged he was wrong and changed that aspect of his interaction with me and his child immediately it would have been a dealbreaker, as I could not live with allllll the hassle and heartbreak of having a stepchild and DH's crazy ex, if he at least did not trust me and placed me as an adult above that of the word of a mere child.

I also think having children of your own tends to muddy the waters more and can cause so many more issues (in many cases) because you now are almost more locked into your relationship with DH due to DS and as you are stretched further due to your responsibility towards DS = less patience, less time and therefore lots more hurt and frustration when your partner is not supportive or realistic when it comes to their stepdevil's!

Trouble is, when you constantly tell your DH you are considering leaving and think your marriage is over, DH is eventually not going to give much credence and attention to your thoughts as you do not follow through.

Personally I do not think you need to learn coping techniques for the fact your DH doesn't respect, trust or listen to you. Those three elements are REALLY important to a successful and happy marriage. The fact DH dismisses you over his daughter is him failing you and DS. HE needs to desperately change and pronto.

Here are my thoughts on how to make DH pay some attention and to seriously consider the fact he may lose you and DS:

Stop threatening leaving or saying your marriage is shite. In fact stop discussing your marriage with DH at all. Showing you are withdrawing your energy from trying to save your marriage while DH is quickly shafting your relationship (meaning you are going to fail no matter what) shows him you are serious and it allows you to divert your energy and attention to making yourself happy.

Disengage from your sd - no more cooking, cleaning, chauffering her around. Your DH throws a paddy about this? Ignore it. Right now you are being torn to shreds by both DH and sd for being involved, for being a stepmum and wife. You might as well get the same reaction from them both for doing sod all for them. Saves you time and effort and while sd is bitching for you doing x and y, let DH deal with the little cow by himself. He doesn't see how she is? Ok, on him, but then he gets the pleasure of her personality and maybe when you take yourself out of the equation of the blame game she will have to find someone else to target.

Do not discuss sd with DH, do not be around sd on your own. Make a life for you and DS separate from DH and sd. If DH wants to treat you like the child and his DD as his wife (believing her over you, is doing exactly that) then you owe him nothing until he realises he made marriage vows to YOU, not his DD. You see when someone acts in that manner, it isn't a signal for you to try more and make up for DH's neglect of you. Its then time for you to reassess your priorities and realise that you need to put your happiness first, to stop putting DH and sd first because they aren't treating you like family right now.

Can you start saving for your own nest egg? Do so, start building your independence from DH. If he sees you being strong, independent and happy then it WILL make him think "holy crap, wifey doesn't *need* me anyone, I may need to start making an effort and listening/making changes in order to save my marriage and ensure DS lives with me full time"

Start realising you are worth more than this and show everyone how you wish to be treated by standing up for yourself in a positive way. So that means, if DH shouts, is being nasty, bullying/guilting you into submission and changing your mind when you withdraw from things then please do not react in a similar manner. Tell him once what you expect from him and how you feel (I feel not you make me feel) and provide exmaples. DO not negotitate, do not insult or be nasty. Be calm and collected, reasonable. Remain on the high round and when you have finished what you need to say, walk away. Do not be drawn into debating things with DH or if he provides promises. He has to SHOW you he has changed before you will revert or compromise on your current decisions.

Good luck!