I (22M) have cheated on my gf (20F) with (22F), what do you think I should do?

I have been in a relationship with my gf for almost a year now, and am very close to her and her family, and we have even got a trip planned for her birthday soon. However, I started texting a girl that I used to go to high school with and it naturally led to a mutual interest, and we hung out once and had kissed, and continue to text, all while things with my gf are seemingly well, she hasn’t done anything to bring this on, and it’s on me and me alone. Neither of these girls know about each other and I am mentally at a point where I want to have this resolved before I begin the school semester soon.

I have been thinking about the pros and cons as I realistically could decide one way or the other, and I’m at a point of not knowing what to do. I understand that I have been a shitty person and I am not a victim here, I just want to know the best thing to do because my gf’s feelings can be very erratic and I am genuinely concerned that the truth would lead her to self harm. I just want to know what to do.

TLDR: have been cheating in a long term relationship and not sure what to do next as the situation is delicate.