I agreed to a polyamorous relationship, but after trying it, realized it's not what I want

My boyfriend wants to be poly but I don't. I agreed to it to make him happy. I don't know what to do anymore because I don't want to be in this kind of relationship but I gave it a try.

It sounds like you know exactly what to do. That's all life really is - trying things to see if you like them. If you like them, you keep doing them. If you don't, you stop! 

You gave it an honest try. You realized it's not for you. So stop doing it! This relationship isn't something you want anymore. You said it yourself. That's a totally okay thing to realize. Now you have some really clear information to act on.

It's possible that your boyfriend is amenable to ending the polyamorous experiment if you tell him "now that we've tried it, I've learned that this really isn't working for me." So that's your first step. If he feels unable or unwilling to be in a monogamous relationship, that's okay - the type of relationship he wants isn't the type of relationship you want, and so you two are incompatible.

The point of dating someone is to learn whether or not you're compatible in a relationship - that's why we don't get married on the first date! You're doing everything right - trying new things, learning what works and what doesn't, and moving forward based on what you've learned.