When I'm asked to give advice to students, the #1 piece of advice I give is this:

Just because a subject is hard for you (math/science/etc), that doesn't mean you're "not cut out for it." You don't have to be a natural at your chosen subject. It's okay if you have to work hard.

There's this idea that floats around, reinforced by movie and TV depictions of scientists, that we're all born geniuses and prodigies. That's not the case. I have colleagues who came into it late, or always struggled. I had to work hard all through school. It's actually normal.

When I was an undergrad, I struggled with a physics class taught by a visiting prof and went to his office hours a lot to get help. Eventually he got impatient and said "some people just aren't cut out for this."

It was super awkward running into him at Princeton during my PhD.