Verysad, you have my sympathy - what an awful situation to be in.
A few things I wanted to post for your consideration:
1). Do you want another baby, or another child? (i.e: toddler / primary-age-child / teenager); babies quickly grow up & you may find yourself in the same situation in a few years again, if you successfully get your #3.
2). Do you (A) need a third child; or (B) a husband more?
2). Do your children (A) need a third sibling; or (B) a present father more?
Please consider my experience with having an unwanted child with an unwilling partner. To cut a long story short, DH decided at 20wks that he didn't want our 1st DD, & wanted me to abort her. I went through the 2nd half of pregnancy knowing my husband didn't want our child, then when she was born, DH resented her hugely; everything was 'the baby's fault'. He also had a very bad reaction to her crying - it infuriated him intensely every time she even whimpered. Consequently I spent at least the 1st 6 months as a new mother trying to persuade a newborn baby not to cry, just to keep the peace - goodness knows how we stayed together.
For the sake of our relationship, DH tried very hard to overcome his problems, & I stayed to let him try, because I wanted us to be a happy family - and 3.5 years later, for the most part, it has worked. HOWEVER....it is not all happy ever after. It has scarred our relationship permanently, and I will always resent him for spoiling what should have been a special time - I do not remember those 1st 2 years fondly. Also, my relationship with DD is not what it should be - I just don't get on with her as well as I do with her younger brother, and I'm sure it's got something to do with the events surrounding her early life. Ironically she gets on a lot better with her Dad than me, sod's law!!
Anyway, what I'm trying to say is forcing the issue will lead to places that are SO much worse than the resentment you are feeling now.
Please take the advice of others & seek counselling - even if it is just yourself initially. Talking through with impartial counsellors really can help, if only to gain some perspective.
At the end of the day, if DH is resolute about not having a 3rd child, then you have 2 choices -
*stay, accept & respect his decision
*leave & find a new partner / use a sperm donor to have your #3.
If you decide to stay, but cannot learn to accept & respect his decision to not have a third child, then I suspect your marriage my already be over - a marriage cannot successfully survive that sort of resentment.
Good luck in whatever you decide & HTH,
Emily xx.