It seems to me that it isn’t the fact of the child that is bothering you. It’s that your boyfriend didn’t tell you himself about this very important part of his life.
There are many red flags in his story: Why isn’t he paying child support? Why doesn’t he see his child? What does it mean that the mother doesn’t let him see his daughter? He may be sweet to you, but to me there are many unanswered questions about his maturity and responsibility. I would think much more of him if he were making a real effort to man up and take on his share of raising his child. It’s possible that your level of distress is that you also have some questions about his character.
I think you tell him that lots of women are happy to be with someone who has a child. What they are not happy to do is to be with someone who lies by omission and who doesn’t take his responsibilities seriously. Since he has these character flaws, he isn’t ready to be in a mature relationship with you or with anyone else.
You haven’t invested enough time in this relationship for it to be this painful to let it go. At 18, you have plenty of time to find a guy who is worth your love. I encourage you to take a big break from this relationship and let yourself be available for someone who is truthful with you from the start.
I wish you well.