24M in a relationship with 25F, now I have feelings for a 23F friend

Throwaway account because people involved might recognize me.

I'm 24M and I've been with my 25F girlfriend for almost 4 years. We broke up for a month, but recently got back together after a very emotional reconciliation. We both cried, talked about everything, and I promised her that I'll marry her. I meant it.

I know I'm an asshole for this, but during the breakup, I realized I have feelings for 23F another girl. I'll call her M.

I've only known M for about a month, but somehow my feelings for her got really strong. At first, I just thought she was attractive, but the more time we spent together, the more I started liking her as a person too. We have a lot in common, and I'm really comfortable around her. We've also gotten pretty physically affectionate with each other.

M doesn't know how strong my feelings are, and I honestly don't know if she likes me that way either. I don't want to assume anything.

My girlfriend knows that I still have feelings for M. She doesn't blame M, and she understands that feelings don't just disappear, but she's scared that M might like me too if I ever confess.

And honestly, I'm scared of that too because I know it would hurt her.

I love my girlfriend, but what I feel for M right now is not something I should ignore either.

I'm having a hard time figuring out how to handle these feelings while staying in my relationship. I don't want to make a decision based only on emotions right now, but I also don't want to ignore what I'm feeling.

Should I choose my girlfriend, the other girl, or myself?