27F, 30M- I’m becoming someone I don’t recognize in this situationship

27F, I’ve been in a situationship with the same guy for almost two years now, and I think I’ve reached a point where I genuinely don’t know what to do anymore.
When this started, it was completely physical. I wasn’t looking for a relationship and neither of us really treated it like one. But after almost two years of knowing someone, talking to them, sleeping with them, sharing things about your life, and having them become a recurring part of your life… somewhere along the way, I developed feelings.

The problem is that I don’t think he’s in the same place emotionally. He’s very inconsistent with his attention. Sometimes he’s affectionate, attentive and makes me feel like we’re actually close, and then he’ll suddenly become distant or barely engage with me. There’s no real consistency, and I’ve realised that the inconsistency affects me much more now than it used to.

When he pulls away, I start spiralling. I wonder what he’s doing, whether he’s talking to someone else, whether he’s lost interest, whether I did something wrong, etc. I hate that I’ve become someone who analyses response times and changes in tone, because that genuinely isn’t how I used to be.
And the frustrating thing is that I’ve tried talking to him about it. But whenever I bring up my feelings or try to have an actual conversation about what’s happening between us, he tends to dismiss them or belittle them. So eventually I end up feeling stupid for having brought it up in the first place.

I know what some people are going to say: “It’s a casual relationship, you can’t expect him to act like your boyfriend.” And I understand that. I’m not asking him to magically become my boyfriend or give me constant attention.

But I also don’t know how to keep pretending that this is still casual for me when it clearly isn’t anymore.
At this point, this situation takes up a ridiculous amount of mental space. I can be having a perfectly normal day and suddenly find myself thinking about him, wondering when he’ll text, wondering whether he’s with someone else, or replaying something he said days ago. Sometimes I spend so much time thinking about the situation that I realise I’ve basically lost an entire evening to it.

And that’s what scares me the most. I feel like I’m slowly becoming a version of myself that I don’t really recognise. I’m constantly emotionally drained, and I already have a history of struggling with depression, so I don’t think this kind of instability is particularly good for me.
But leaving is also incredibly difficult. It’s not as simple as “he treats you badly, so just leave.” I’m attached to him. I care about him. There are good moments, and those moments are probably part of why I keep coming back. And after almost two years, walking away from someone who has become such a significant part of your life feels genuinely terrifying.
So I’m stuck between trying to somehow detach and continue keeping this casual, having one serious conversation with him about what I actually need, or accepting that I’m too emotionally invested for this arrangement to be healthy for me anymore and walking away.

I would really like to hear from people who have actually been in long-term situationships like this.
How did you know when it had stopped being casual for you? Did you manage to genuinely detach while continuing to see the person, or did you eventually have to leave? And if you walked away, how did you actually make yourself do it when you were still attached?
I’m not necessarily looking for “just leave him.” I know that’s an option. I’m trying to understand what the healthiest way forward actually looks like when you’re emotionally attached to someone who doesn’t seem to want the same thing.