different name for this 
I think I'm basically repeating what everyone else has said but OP, seriously, if you know in your gut that you do not want another child - dont have one. Its not fair on the child, you, your dh or current children.
I do recognise and understand the position you are in though, and its very difficult when you both have different ideas on how many dc you want but, I suppose, your dh will just have to be content with the 2 you have.
I also think Acanthus makes a really good point - I know a lady who agonised over having "just one more" (after a large gap from her first 2 dc). She had triplets. nuf said. She now tells everyone to be very wary when considering "just one more..."
No one can tell you to have another or not. Everyone has different lifestyles/ coping abilities/ energy levels/ support/ finances/ patience etc
Im going to go completely against the grain here though - for anyone who is expecting number 3 and is now terrified after reading this (bearing in mind that I fully accept that our number 3 is just 5 months old so things may change/ get harder):
I adore having 3. I absolutely love every single last second. Im less stressed and more content since number 3 came along. Ours are (just) 6 years, 22 months and 5 months. I cant possibly imagine "just" having two. I often catch myself smiling to myself when I pass 3 bedrooms with 3 sleeping children.
I dont find it hard - at all. My dh for various reasons (work and an unexpected opportunity) has barely been around since number 3 arrived (not his fault). I've had to do 95% of all childcare/ housework - both day and night. Im back at work (since last week) and have to get 3 out in the mornings on my own - CM and school drop offs and collections etc.. and.I.still.love.it.
Everyones different - dont let this terrify you too much