I am sorry you hurt.
Let me ask you this... Set BF aside for a minute. Did you feel better dating in a monoamorous and monogamous framework?
If so, maybe it's time to come to terms with the fact that you are "monoamorous AND monogamous" and that's how you prefer your romance shapes to come in. NOTHING wrong with that. You have tried. You are not "monoamorous but ok in a polyshipping framework" as a "V" arm to a polyamorous hinge person. It is causing you disturbances in your mental health and emotional health -- stress and heartsickness.
Everyone has the right to have their romances in a shape that they can be happy and thrive in. There is no fault. It does NOT lie with you. It just is what it is -- nobody's fault that you have different values and romance shape hopes that are not compatible together. Could feel sad, could feel disappointed. Could also not beat up on yourself. Treat yourself more kindly and with more self respect. That is what the dating time is FOR -- to find the compatible ones.
I'm sure BF is very nice, but you sound like you have basic imcompatability issues here. He is polyamorous and values being in polyship with several women. You do not want to be -- you prefer the Disney/American traditional dream values that imply a romance that is one on one.
Could stop trying to square peg round hole because YOU are responsible for your best healths -- physical healths, emotional health, mental health, and spiritual health.
If you were whanging your face with your cherished frying pan, and announced you felt yucky... I'd tell you to stop whanging. Love your pan a different way. Make pancakes in it instead of whanging. You are whanging your brain and your heart with the BF who has different values than you. Neither is better than the other -- just different. And not compatible. You say you are tried of feeling this way. So contemplate changes.
I'm going to gently suggest you consider breaking up. Continue to love him in a different shape -- as friends. There is NOTHING wrong with a good friendship. It is hard to imagine loving a new person -- because it's hard to know what is not yet here. That's normal.
I'm sure he will always be "a love of my life." But Life is long, and he doesn't have to be the "current active love" of all
your life if there's basic incompatible here. If for your own best long term health you have to go through some short term break up suckage -- that could be the better investment for you.
If you choose that, you could go through the stages of grief
post break up. You actually could be there already in the "denial" or "bargaining" stage as you come to terms with this basic incompatible. Could spend some soul searching time to sort your inner feelings out.
It is ok to be a young person. You will naturally outgrow it and move to your next decade of life. You will NOT outgrow being in a relationship where you cannot thrive and be happy in. You have to actively do something about it -- change the relationship.
I am sorry you feel trapped. You are in a Time of Discernment. That can be uncomfortable for a young person if you have not traveled that road before. It comes up many
times in life -- your values get tested and reaffirmed or reshaped by Life. Sometimes you have to make a call you don't want
to make but have
to because it is the healthiest thing for all concerned. So the heart is heavy.
You will be ok. Hang in there. Do the job in front of you -- sort yourself out and decide whether or not to stay in this polyship.
I think for your best health, the best thing could be to change it to a friendship. You have already been here a while trying it out. How long you stay in the dating time before you determine a partner is a long haul runner or not is up to you. (You sound like you want a long haul partner.) But to me you sound like you could be done, and trying to come to terms with that disappointment.
It is not fun. But it is what it is. Be brave and do the job in front of you -- sort yourself out.