My partner is making me feel bad that I’d like to stay monogamous with them and have our romantic relationship be that one. They talk about having to use daydreaming to cope and read fanfiction as well and will do it with me there when I want to try and do stuff with them like hanging out and relaxing together. It just makes me uncomfortable that they have to do it to cope as they put it and we have discussions but it more and more seems to be a “you have to accept this” situation.
If someone asks you for something, and then won't take "no" for an answer, they weren't really asking in the first place.
It's one thing for a partner to ask for a compromise, like that you be okay with them indulging in polyamorous fantasies and fanfiction. But spacing out, ignoring you, or getting lost in any media while they're hanging out with you isn't cool. Ask them if they could put the phone down or try to be more present to you during your time. If they can't or won't, then that's information you have about what being in a relationship with them is like.
I've said before that I really dislike the phrase "cope with" as it relates to a partner's needs or behaviors. If your partner is treating you like something they have to find ways to "cope with," that's an unfair and unhealthy framing. Performing how miserable they are in the terms of your relationship is manipulative, and no one should be guilt-tripped into polyamory. I'm of the opinion that living in resentment is never good, and if you make a decision, you should make it wholeheartedly, and commit.
One of my partners is terribly allergic to cats, so I can't get a cat. If I did, he wouldn't be able to come over to my house, ever, or even cuddle with me without me showering and changing first. This bums me out, because I like cats - but I've decided that it's worth it to stay with him. So I live with that decision, and don't wallow in the "what-if"s. I don't show him photos of cute kittens and say "see, that's what I could have if it wasn't for your issues." I don't bemoan my cat-less life. If having a cat was that important to me, I could leave the relationship. I decided not to - that was my decision, and I need to own and live in that decision. I can be privately bummed out that the stars didn't align for me to have this relationship AND a kitty, but I made my choice, and it's best to move on and live in the world that exists.
So, you're within your rights to ask your partner to drop this issue, to stop guilting you and making it seem like your relationship is this terrible psychic burden they must cope with. If they can't or won't, you should probably leave the relationship. It's not fair to either of you to stay in this situation, and you don't deserve to feel like a problem to be dealt with.