Hun, like you, I had my youngest daughter via donor insemination after my marriage had broken down...then my ex wanted responsibility, and now there's a hug mess.
Anyway, my kids are 5, 4, and 20 months, and it is very hard, probably because my 5 year old has special needs and is more like a 3 year old in many ways. Her meltdowns are the most difficult, and then the constant attention she needs to make sure she doesn't eat something that could hurt her...and then the temperature regulation issues.
Anyway, other than her medical and developmental issues, I honestly wouldn't change anything. I think the hardest part personally is trying to recover from seven years of abuse. I find myself still, occasionally taking on his personality, but I think my kids can see I'm trying very hard to be a better mom. My son is very understanding, anyway, Then, I'm working full time and supporting them at two houses.
But overall, it is so damn rewarding. To watch my kids play together, protect each other, make up songs and games, and then hugs and kisses and love times 3, it is the best gift ever. When I kiss my son goodnight at night, I tell him he's my favourite boy, and he tells me I'm his favourite girl. It has gotten easier, now that I'm in the routine. Once you get through the exhaustion of full time employed AND full time mom, the benefits far outweigh the negatives.
And given your history, I'd start trying now, especially if your donor is willing to try. I had my tubes tied after my youngest, but I'm having them undone in the next few months, and her donor is willing to donate again too, so she has a full sibling.