It took about 3 months for me to feel I was coping. The number of times I rang DH to check he wasn't going to be too late home...
Try to focus on the successes, on the little things.
Do things with the little one, but if necessary reward yourself too: I'll play this messy sand based game for 20 mins. Then they can have a little TV while I have a cup of tea.
I found structure really really helped, plus getting out of the house every single day, rain or shine. Toddler groups. (age dependent of course).
You don't have to be perfect, just 'good enough'.
Make sure your DH is doing his bit. We used to alternate bedtimes, even though DD2 used to cry and cling on to me every time DH took her (normally laughing by the time they were half way up the stairs).
Post adoption depression is a real thing. If this might be you please contact your GP.