Do you think I should break up with my actively suicidal girlfriend after she admitted she was underage to me
Yikes, that's heavy. What's the current situation now?
I don't want to be a babysitter anymore
A relationship is based on trust and understanding, and she broke that trust by lying to you about her age. Although you cannot see each other romantically, you can continue the friendship and try to get her the help she needs for her suicidal thoughts.
She knows she fucked up and I don't care about her.
If that's the case, I the best approach would be to contact an adult and get her help. Even though she broke your trust, she is a minor in a bad place and needs professional help.