OP - I was in the same situation, effectively, as AnnaFalactic. I was 18, became pregnant and the guy was not interested one iota. Haven't seen him since I got pregnant and he's had nothing whatsoever to do with us.
Having said that, having DD was the best thing I've ever done. She's eleven now, I'm also now married. Looking back, it was difficult and could be very lonely bringing up a child alone. But, there are a large number of positives that I appreciate even more now that I'm married. Being a lone parent by choice means that you have the autonomy to make all parenting decisions. You can choose the name, the way of feeding, whether to co-sleep or not, what kind of a routine you want. I co-slept with DD and would love to co-sleep with any future children, but DH would be adamant that they should be in a cot.
I didn't have to juggle a partner's needs with mine or DD's. I didn't have to feel torn between being a wife and a mother (as I do now). I didn't have to worry about housework, cooking meals for other people, making sure everything was tidy (as many, though of course not all, women have to do in relationships). I was able to focus totally on DD, enjoy her, be with her.
If you have the option to get great childcare (a nanny for example) then being a single parent isn't really any more difficult than being married. When DD was ill and I was working, we had an au pair that would be at home with her. Things like sports day, parents meetings etc could be slotted around work if necessary, in the same way as if both parents worked.
I think, for me, the most difficult part of being a lone parent was weekends, when all my 'mum' friends would be home with their partners and I would be alone all weekend. Weekdays, when DD was small, were fine - there were toddler groups, NCT groups, playgroups, coffee mornings. Weekends, though, were awful. But, if you have a good circle of friends then that could be overcome. In my situation, friends my own age were off on their gap years or at university and the friends with babies were at home with their husbands. Now, however, if I were a lone parent, I think it would be easier as I know a far larger group of people.
Fundamentally, for me, having children was always something I wanted. And I want more children. Things are rocky with DH and I, meaning that I may very well be in the same situation as you, in which case I would, without hesitation, go down the sperm donor route. I know other women that have and they don't regret it.
Good luck and feel free to PM me as I've done quite a bit of research into the donor idea.