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So I met my best friend a little over a year and a half ago. We both work at the same place and hang out outside of work and go to the same college (we were basically roommates). I was originally “in love” with another guy whenever we started hanging out, and he helped show me that he was toxic for me. After that, I was able to get over that guy and focus on myself. Doing that, I realized that I actually had feelings for my best friend. I thought it was nothing at first, but throughout our friendship, those feelings have grown a lot stronger.
We both love Star Wars and Marvel. He makes sure I know that I am invited to different events that he is going to and tries to include me in everything. We go on late night drives (just the two of us) and listen to music or talk if either of us are upset or overwhelmed. People always know that he rides shotgun in my car because he designated himself as my co-pilot. I have had a few people ask me if we were dating because of how we act with each other.
The reason I haven’t told him yet is because he has a girlfriend. This hasn’t been a long-term girlfriend, but it’s been going well for them so far. He’s fallen for both of my best friends in the past and I’ve always been there to give him advice about any of the girls he was thinking about dating. Currently, his girlfriend and him are doing well (other than one night that they got in a fight and he ended up crying and talking to me about it).
I’ve had the overwhelming urge to tell him lately because my feelings are growing so strong and it hurts me a lot of the time. He’s also been teasing me a lot lately (jokingly) and is always cracking inside jokes with me which hasn’t helped. His girlfriend is so great and I love her. I’m just scared that I’m gonna go insane if I don’t tell him. I also don’t want to lose him as a friend because I don’t think I’ll be able to handle it. People keep telling me to let it run it’s course and I’ll get over it, but I haven’t.

What should I do?