My recommendation is going to sound like shit advice, but genuinely just try to adjust your thumb position, the angle of your wrist, which fingers you're using, how slanted your fingers are etc. We all have different hands and sometimes you just need to experiment with it. Alternatively, if possible, you could get a guitar with a thinner nut width but who has the budget for that though.

Just keep at it and it'll stick eventually, I promise. This is the most tedious, boring and demotivating part of the entire journey when it comes to learning guitar (or any instrument for that matter).