Totally not wrong to feel that way. You were bullied into something that you do not want, and every time you try to express that - from what I'm gathering - you're being brushed off and ignored. It almost seems as though your feelings in this don't matter, and that's not right.
It's very difficult to see the positives in something when you don't want it. And how long are they willing to wait for you to "see the positives?" How long are you willing to keep trying to force yourself to be okay with something that you don't want? You say you don't want to let this break your relationship, but what if, in the course of all of this, you begin to resent him? Being forced into something has a tendency to breed resentment.
If she says you don't have to be romantically interested in her for it to work, then it isn't a triad. It's more of a vee with you and your friend as arms and your boyfriend as the hinge.