It's OK! It's OK! I know this is easy for me to say and for you to do, but try not to stress too much about it. I think the thing that will help the most is to really think about why you're not with the father of your baby. Typically, the reasons will be very good ones, ones that would mean if you'd tried to stick it out the relationship between you two and between you and your baby would be much worse off.
I think what I hear the most about people who have rough relationships with their kids, is more about communication with their kids, trust issues, stuff like that. And you simply can't stress about that, because it's gonna be too far in the future. Just do the best that you can, take it day by day, and just try to keep being there for her.
As far as telling her about her biological father when she's older, my personal opinion is to be honest without being derogatory toward the father. It could be as simple as "your father and I couldn't live together" when she's young and leave it at that. These days, it's really common to have a one-parent family, so it's not like it was years ago when people who came from one-parent families had this huge negative stigma to deal with.
But you simply can't afford to stress about 'what ifs' for stuff that's gonna happen years from now! :) You'll just make yourself crazy! Try to do something nice for yourself to reduce your stress, come to peace with your decision regarding your baby's father, anything like that. The stress you're putting yourself through now is only making your life miserable and isn't great for your baby, so try to find something that will allow you to come to peace with your decision, ok?
*hugs* Hang in there - I'm sure you'll do fine as far as your baby is concerned!
J.