Quick context because you’re all strangers—lovely strangers I’m sure, but still:

I love my fiancée more than anything in this world and the next. She’s been the most supportive and encouraging and loving partner I could have ever asked for, and I still can’t believe that I was fortunate enough to find someone like her.  

The specific situation I need advice about is really just from how the past little while has been kinda rough—I’ve been having some not good mental health days, and she’s been experiencing a lot of emotional fatigue, and the general vibe of a lot of our conversations lately has been far less comfortable and happy as a result, often added to by me overthinking and making the situation worse on my own accord.

And I know that so, so many people talk about normal relationships having conflict, and that that’s something to be aware of and work through. But, and here’s the main thing, I can’t help but feel like it’s NOT normal to worry that she’ll *leave* when tension or conflict arises. That just one argument will suddenly be enough to make her decide it’s not worth it, and in the end it’ll be all my fault that I’m alone. That anxiety can’t be a normal thing, and certainly not a good thing.

Usually it’s spurred on by intrusive thoughts of her leaving me, which could be due to a possible case of obsessive-compulsive disorder on my end (diagnosis pending, take that with several grains of salt), but in the end I still end up scared that that one argument is enough to ruin the whole thing, and it’ll be all my fault.  

Does anyone have any advice on how to avoid that fear, and to give myself more confidence in the relationship when things aren’t perfect?  
Hell, the last thing I’d ever want is that fear of things ending to be the thing that actually breaks it, yknow?  

(Also in case anyone is theorizing astray, I genuinely have no desire whatsoever to end the relationship—I truly do love her to hell and heaven and back, and it feels like this fear is almost a foreign thing? Like it’s completely and entirely separate from my true feelings about the relationship.  
I experienced this kind of thing with an ex before, but that time it was also added to by doubts about the relationship already? But this time I couldn’t be more sure about how much I love her, which kinda makes it even more confusing and frustrating tbh)  

Any advice would genuinely mean the world—I love this woman with all my heart and am so, so excited to marry her, and I’m determined to make sure that I’m not jeopardizing it with overthinking and unhealthy responses to anxiety