It does seem so unfair, I agree. Some people just seem to have more to endure than others, and I can't imagine having gone through so much. I'd probably have given up long ago myself, so I really admire your grit.
I don't think it's you doing anything wrong, and certainly not your fault ... but by persevering, you're showing your daughters what you're made of. They look to you for the love, and they do get it by the boatload from you. And, Lord knows, none of us is perfect. We all stumble along the way, but someday you'll look back, and I don't think you'll regret how you've done it because you've tried your best.
There will be good times, too. Just keep an eye out, and don't let opportunities slip by. Sometimes they come along in the most unexpected ways and places, when you're not looking for them.