I keep thinking about my GF’s friend. Anyone that has been in this situation?

TL;DR: How do I stop this thought process? Please share some experiences.

I \[M25\] am a guy in my mid-twenties. Got a 3.5-year-long happy relationship with a girl \[F27\] who I very much like to consider to be the future mother of my children. During the past couple of months, her best friend has become single and has been dancing in my thoughts.I consider myself to rational in situations like these. I have never had the urge to cheat in a club nor in a situation like this, because it has always been easy for me to rationalize that the girl I am with is going to make me happy. This is why I agree with many of the comments in this older post (can’t link) about working on your relationship and committing to it because the grass is greener in your relationship if you work on it.

But here is the problem, I am now in a situation where I am constantly confronted by the friend of my GF and her nice personality. Both my girlfriend and this friend keep inviting me for things to do together. I try to decline most of these offers because I have the feeling that the friend flirts with me from time to time. This keeps me in a constant loop where there are weeks/days that I don’t think about this, but where this feeling comes back when I see the friend again. Although I can control my actions very well, I haven’t found a way to control the thoughts in my brain. It feels like I am cheating with just my thoughts, thinking about what a relationship with this friend would look like and if it would be better than what I have.

I welcome both advice and experience sharing. What did you do? Did you tell your spouse, confronted the friend of your spouse, or completely cancel everything that had something to do with the friend?