You know, when I get feeling like that, I remind myself that I can never be everything or do everything 'right' for my kids. In fact, I joke that no matter how hard I try, my kids will still probably end up on Oprah one day saying how they needed "x" from their mother and never got it!!!!
I use that for a little humor, but it does remind me that I can't be everything. I give them what I think they need and that's all I can do.
I know that when they are little we do the bulk of the parenting alone, but as your kids get older, there will be more people who will help 'raise' them. You won't be alone. You may find that family, friends, other families, neighbors, church leaders, group leaders, teachers, etc. may one day have a connection with your children and will help guide and nurture them too.
I know it can feel overwhelming at times. You are doing a great job with everything you've had to endure. You are more than enough for those two little ones. You will do the best you can and they will love and appreciate that for years.
Just had to add that I love what meemee wrote.