Im in love with my best friend who’s in a relationship

My best friend (24m) and I (24m) became best friends in high school. Eventually I got feelings for him and it got to a point where I couldn’t be “just friends”, so I told him the truth. After about a week, he told me he thought he might have feelings for me too, but he concluded we could only be friends. We also needed to take a break from being friends because everyone in school thought we were dating, and he didn’t like that much. Eventually, we became close again, but our friendship became on and off throughout senior year and college. However, there were times where he’d flirt with me or he’d get jealous if I was dating/ talking to someone (post-revealing my feelings).

Now, about 8 years later, we’re talking again, my feelings have never changed, despite dating others, and his behavior is almost the same as before. Though he doesn’t flirt as much, he seems to want to remind me of my attraction to him. He treats me differently than his other friends and there are so many moments when we hang out where it just feels romantic or angsty (staring into each others eyes and just smiling, slightly making physical contact with our arms, finding ways to spend time together longer than we intended, etc.)

It all sounds perfect, but he has a girlfriend (23f) of about 5 years. Only thing that seems strange to me is he complains about her often and he seems to keep me a secret from her (ex. when talking on the phone, he’ll just refer to me as a “friend from school”, even though he refers to his other friends as “my friend \[name\]”). There are several other indicators that confuse me on how HE perceives me. So Im curious on if I should ask him, if I should tell him how I feel, or just keep it to myself forever?

TL:DR, Im in love with my straight best friend and he is in a hetero relationship.