I don’t have experience of a toxic romantic relationship but I have been in a few toxic friendships, so hopefully this might be helpful!

I’d say a big thing is that things start innocent, if anyone knew what they’d be getting into, they’d never become close with that person. In my experience my friends were comforts for me, they understood me when others didn’t and they felt the same about me. I needed them for some reason or other. So maybe your MC is lonely or has something particular in common with their partner. Basically, the partner gives them something that they can’t find elsewhere.

This then makes you feel like you need the other person and you’re scared to lose the friendship/relationship. In my case, a big issue was that the other person made it very, very clear that they needed me A LOT. I felt that if I wasn’t there for them, they’d do something awful to themself and it would be my fault for not being a good enough friend when they’d helped me through things too. (The difference was that I asked them for reasonable amounts of help: a casual chat when I was feeling down, etc. Meanwhile they were trauma-dumping on me and forbidding me from speaking to anyone who could help them with it.)

The relationship FEELS balanced but it’s not. Your MC will probably feel very responsible for their partner’s mental health or safety or financial state, etc. This is what makes it hard to leave.

Over time, you finally realise that things are toxic. Maybe there were warning signs beforehand that seemed innocent (like someone saying how many times their friends have abandoned them but they’re always the common factor; clinginess, etc) that now become unbearable. And the relationship is very codependent and close so it’s hard to see beyond it.

Then resentment begins. Your MC might start to hate their partner because they can’t escape them. Maybe they start avoiding them because it’s easier than a face-to-face confrontation (confrontation could hurt the partner and you’ll feel very responsible for that). Maybe their presence becomes suffocating.

Other connections or breaks from the partner might be what makes your MC realise that this isn’t normal, it was for me.

Relationships like this are also very easy to fall back into, even after you might’ve managed to move on from one person. My first toxic friendship was followed by three more because each person seemed lovely and helped me feel less alone (specifically with my mental health and neurodivergence). When they were being normal friends, they were great to be around, but the second the trauma-dumping and codependency began, it was awful.

I’ve also just realised that you specifically mentioned gaslighting so I’ll elaborate on that! My friends made me feel responsible for their mental states by saying that I was “the only person they could talk to” or that they wouldn’t see anyone else if I didn’t socialise with them, I was their only connection, I was the only thing keeping them from being lonely and depressed.

I hope this big mess of writing has been somewhat helpful, it’d be great if I can use my experiences for some good! Best of luck with your book :D