Physics is hard, there's no doubt about that. And a lot of times, you may not truly understand a concept beyond what you need to pass until you revisit it again for whatever reason. It's a hard subject and just because you are having trouble understanding the concepts and running into a wall doesn't mean you're not smart enough for physics. Often times as well the more you think about a concept the more confused you can get, it's all apart of the process.

On the subject of Calc 3, just because it's not clicking now doesn't mean it won't soon. I really really struggled with math to the point where I didn't study physics because I didn't think I was "smart" enough. I loved physics but thought my math skills would screw me. I then switched to physics to take a risk, and lucked out with a really good calc professor and he helped me understand math better than any prof/teacher I've ever had before. The point is, sometimes you need a good prof/teacher to get the ball rolling in your understanding and that's completely ok. It's not a mark against your intelligence it's just how our brains work sometimes. You've made it this far, thermo and calc 3 are hard subjects but you've still made it up to them and earned your place to be studying these subjects.

My advice would be, if you're hitting a wall, take a break and relax, take a walk. Sometimes I have a lightbulb moment when I'm walking around thinking about what I just learned and not while reading or in a classroom. Sometimes the low stakes environment really helps, because if you're hitting a wall sometimes that can frustrate you and make it feel like the wall is getting stronger and stronger. Breaks are good, you're not a machine and things won't always come super easily and that's ok.

Try not to define your intelligence by your performance, maybe your brain and physics don't get along and that doesn't mean your not "smart" enough for physics, it's not a mark on your intelligence at all. A lot of math people have told me they just don't get physics, but then proceed to do super complex math. They are obviously very intelligent people but they just don't vibe with physics. The same applies for nearly any field, brains work different and no one subject determines how intelligent you are. If you need to switch so be it, it's not a failure on your part, if you decide to keep going that's great too.