OP, thanks for clarifying, and I'm sorry that you may have fertility troubles.
I think that BertieBetts raises some very good points about what being single and being responsible for a child means. It's not just about there being no-one there to change nappies (changing nappies isn't that difficult anyway) but how things are when the buck stops with you.
Also fourteen is right as well - it's a question of when you give up on what you think is the 'ideal', bearing in mind that if you wait for that ideal, you may find yourself unable to have children.
It's very hard, but if I were in your shoes, bearing in mind that you still have a few years to play around with, I'd maybe do some contingency planning. First, cost. What would it cost to go down the donor sperm route, what would childcare etc cost, could you afford it, all that kind of stuff. Second, fertility - could you get yourself checked out at the GP to see what options you do actually have, what possibilities are open to you. Third, support - and be realistic about that. What do you have around you, that you can rely on, and that you would happy to be relying on?
Who knows, you might meet someone fabulous in the next six months and get married and have kids with them. In which case, great, and you can ditch your contingency plans. But if you really want to have children, and you know your time to have them is limited, would it maybe be a good idea to start thinking about how to do it?