i went through the same thing. I was 34 weeks pregnant with my son, and my partner left me. I rung mum straight away and moved there that day, but that night i went into labour, the drs stopped the labour and gave me steroids to help my son grow. (btw im 18 myself) after i got out of hospital, i spent all day every day with my ex, i went to work with him, i even stayed at his house every coupla nights. But the whole time i had to listen to him talk about his life, and it was hard because although i was a part of his life, i wasnt one of the most important things in it anymore. I had to watch him look at other females, and talk about one of our friends staying over when i didnt and how she did this and sghe did that, but they werent dating. This put a lot of stress on me, which was extremely bad for the baby, my bp sky rocketed and i got very sick from the stress of it all, but i thought if i stuck around him long enough, hed come back to me.
I too didnt want to give my son his last name, and i didnt want to share my son with him, i knew it was ours, but i thought he didnt dseserve to be a part of it because he was the one that left.
Basically i guess im saying its ok to be a little selfish, BUT dont live with him and dont spend all your time with him, give him space to miss what he had and he might just come back. Its completely your choice, but thats my opinion.