I think I might be poly and I’ve mentioned it to my boyfriend before and he’s really against me being with anyone else cause he gets really upset and I just don’t know what to do because I think I would be happier if we had an open relationship but I really don’t want to hurt him

There are no magical words to say to someone who is “really against” polyamory to suddenly change their mind, unfortunately. Your boyfriend has every right to refuse to be in an open or polyamorous relationship. That means you have a choice to make: stay with your boyfriend and be monogamous, or leave your boyfriend to pursue a polyamorous relationship. Both require a sacrifice.

Sometimes we find ourselves in situations where we can’t get 100% of what we want. You don’t want to hurt him, but you also don’t want to stay in a monogamous relationship. Only you can make the call between those two - but it doesn’t sound like there’s a way to get out of making that call.