polyamory

I have been openly pansexual for 6 years. I have dated girls, guys, nonbinary, trans, you name it.... but only ever one at a time. about 3 years ago I was introduced to polyamory. I thought it was amazing that three people I was great friends with could be in such an open and loving relationship. though I didn't think I was. I've never had had strong feelings for more than the person I was with. so I just things go on and maintained my love and support for my friends.

flash forward to now and I am 22. I have been dating a man for about 6 months and I love him dearly, we may be expecting. but I also have romantic feelings for a girl my best friend introduced me to... I don't wanna bring it up to him for fear he wont support it. he doesn't like that I have slept with my best friend(even though it was years ago and there was no feeling to it). I feel extremely comfortable with my sexuality and after developing feelings for the girl I feel as though I would be willing to try a poly relationship but he wont be. I love him and I don't want to lose him but she understands me on a whole other level. she is also pansexual and she and we have so much in common. I don't know what to do.... HELP