I told the girl I am interested in that I am poly and she didn't take it well. I feel heartbroken now and wish I could change me, so the people wouldn't always leave me as soon as I tell them about the way I am hardwired. I also get that it just wasn't meant to be but I'm still a sad mushroom now.
I am sorry you are feeling like a sad mushroom. It is okay to be sad and frustrated when something doesn’t work out! All of us face times when we don’t get what we want, and it’s okay to admit that it sucks.
I would estimate that about 2/3 of the time when I am interested in someone, the poly angle means things don’t work out. Most people identify as monogamous, so the odds aren’t exactly on our side. (One time, a guy and I planned a romantic weekend in a tiny Irish seaside town, then stood me up because he realized between agreeing to meet me and actually meeting me that he wasn’t actually okay with it! I spent the day wandering on my own, collected some seashells and pebbles, and then took the late train back.)
There are a few things you can do to improve your odds. One is seeking new partners and dates within the polyamorous community. Another is, if you can, being more out and open about your polyamory in general with your friends and social circle, so anyone who might be developing interest in you has that as pre-existing context. And third, think about how you are telling people you’re polyamorous. If you act like you’re unburdening a great secret, or if you spring it too late and people feel led on, that’s more likely to end poorly. Practice being upbeat and chill about your polyamory and try to be patient.
In the meantime, have some ice cream and let yourself be sad about your feelings not being returned. That always feels crappy.