I sort of did choose to be a single parent - from the minute I found out I was pregnant (at 18) my DD's father was useless - one minute he wanted to be with me and be a family, the next minute the baby wasn't his and he wanted nothing to do with us. He kept changing his mind constantly, then when it was definitely over, he beat his new gf up, not to mention the fact that in the short time we were together he cheated on me and got her pregnant aswell!
Long story short, I decided that he wasn't the kind of person I wanted in my child's life. I didn't have family support at the beginning, my Dad wanted me to terminate, but eventually came round to the idea, and at that age, I drifted from my friends because I couldn't go out and party any more.
Different story when DD was born though - my Dad absolutely adored DD, my mum was supportive and so were wider family, I regained friendships etc. I lived at home til DD was one, then got my own place, then got a saturday job, started a degree with OU and now have a part time job in my chosen career field.
The hardest part is when you're ill and you still have a child to look after and a house to run, but I'm still lucky to have good family support. Working definitely keeps me sane! I can raise DD as I choose, my child, my rules, don't have any aggro regarding contact, maintenance etc (don't get any!) I'm lucky because money isn't an issue - even though I'm working there is a lot of financial support for lone parents.
So while you might find it hard, it will be do-able - especially if you go down the sperm donor route - you won't know life to be any different as you won't be going from the nice family life in a relationship to being a single parent with all the hassle of the ex etc!