Do you know this sounded like me a couple of months ago. I was posting issues about my husband on here and about his kids - I even answered a query the other day about an 18 year old step daughter.
Long story short - yes, my husband can be difficult and yes, my stepchildren are pretty horrid at times and yes, my husband eternally makes excuses for them. But it was me that was going downhil - I ended up at the doctor's for depression and six weeks on I am not getting stressed about the stuff I used to, I can take or leave my stepchildren's rudeness or behaviour and I don't get myself twisted into knots about "being blamed" for everything that goes on in other people's lives.
I'm not saying it's an issue for you - I mean depression - but I know I was taking the weight of the world on. Now, I brush off remarks and smile - I can't remember the last time I had an argument with my hubby.
Good luck though - I've been there ...