Damn, I know how you feel I usually just ghost the people I'm in a relationship with, so I don't really know what you should do

-tries to think of a solution but realizes that it is outside my ability-

If you are in a relationship and you fall in love with someone else, then that probably means you dont like your partner anymore

If he's truly just a friend to you, he would respect your decision- sometimes hurting someone's feeling is inevitable, but you can come out of that situation knowing what you want and you go after it