My XH did something similar when DS was 18 months old. found himself a younger fitter model with no responsibilities.
I think a lot of men don't appreciate from the off how hard domestic life is gonna be.
Anyhow, my advise is to surround yourself with as many friends and family as you can, what ever you feel right now is normal. get to the doctors if you can as well - I didn't until 4 months later, and put on a brave fact, 6 months after that was when I admitted defeat and got some anti D's.
anyhow at the time I felt like you - DS is the reason i'm here now I firmly believe that, without him I would never have survived the dark days.
You can do this thou, and you'll find an inner strength you never knew was there.
I personally found that I had to detach from XH - moved out for 5 weeks and barely spoke to him while I got my head in order. changed totally in every sence of the word, and developed an inner confidence as well.
It's hard work, I won't lie, but it's so worth it now when I look at DS and think you're fantastic and it's my hard work, or like this w.e just gone look at XH and think you're totally pathetic.
fwiw if it helps we're 2 years down the line, OW decided 5 months ago she didn't want to be around him any more and he's been desperately trying to salvage something - tried it back on again with me, something that is NEVER gonna happen, but I really do think that 90% of men do these things and hten see how well we cope and realise exactly what they've done and want it back.
Anyhow bit of advice I was given...... Be like a swan - elegant and beautiful on the surface but paddling like hell under teh water to keep afloat. You are a beautiful swan never forget that. xx