Firstly, I don't think you're selfish, not at all.

Secondly, I'm a single mother, and I would not want any man building a relationship with my daughter until marriage was on the table.

I have a steady bf, but since our future together is so uncertain at the moment due to career unknowns (he may need to relocate and move abroad) we have not encouraged a relationship between himself and my daughter. She knows he exists, they spend some time talking and whatnot, and even though there's a fondness on both sides, they don't spend any one on one time together.
We don't do "family" type activities together and neither of these things will happen until further notice.

I keep them as seperate as possible as I don't want her having to go through losing another father figure at such a young age.

My point? I think you've been smart about it so far. I know its not how things are commonly done in this world full of us single mothers, but I think its the safer way to proceed when a young child is involved.

I can only imagine what it must be like for those kids who's mom encourages them to get used to the man she dates, forming a bond with these guys and then having to adjust to them leaving as well.
Rinse and repeat.

I couldn't do that to my daughter.

I also think its very good that you're so self-aware, at such a young age. To know you're not ready to settle down into family life at this point. Many a folk jump in head first, drunk on love and go "wtf did I get myself into" down the road when they realize they're missing out on things they would've otherwise chosen to experience in life.

So my suggestion?

Talk to your GF, ask her what her goals are with regards to your relationship timeline.
If for instance she says "I want to be married in the next year and have another baby by the time I'm x years old..." then you in turn communicate your timeline.

If neither of you can agree on a compromised timeline that suites both of you, then its time to move on.

Because eventually either she will feel like you're dragging your feet and become resentful toward you, or you will feel forced into rushing along and resent her and her child.

Communication, it saves relationships, or on the other hand saves people from prolonging future agony.