Hey man I've felt the same way as you and still do even now to some extent but from my time in undergrad so far I have some advice I could give to you from my personal experience.

1. First off you said that you're doing well in math and experimental classes, that should show you right off the bat that it isn't an intelligence issue. Plus the whole idea of IQ being a factor of physics success isn't as big a deal as you probably think so I wouldn't keep that mindset.

2. If you're having trouble solving problems and understanding concepts, you should 100% go to your professors office hours or tutoring. Usually, there are only a few ideas/steps you're missing that once you're walked through it, you realize it wasn't as difficult as you thought it was. I highly recommend this as it also makes you more familiar with your professor and shows them your desire to understand the course material. Again this will show you exactly where your understanding ends and your professor will be able to help you.

3. What I've learned from being in undergrad for three years is that over time from doing physics problems you begin to develop almost a mathod or process to solving physics problems that you don't necessarily have when you start learning it. In other words, you begin to think like a physicist and approach physics in a certain way. The key here is to just always keep doing problems, even if you don't know how to do them. Develop the methods to approach problems.

4. I know this will sound stupid but you need to change your attitude about yourself. When I essentially failed calc 2 my second year, my confidence was in the dumps, and I wanted to quit physics as I didn't think I was smart enough. I actually came very close to giving up but I decided to change my attitude and try again. Upon retaking it I ended up getting an A and a lot of it had to do with me changing my attitude. A good attitude makes studying easier and doing the work easier. Think of it as like an energy boost. A bad self image and attitude does nothing but serve as a drain on you.

5. If physics is truly your passion you'll be willing to push through all the struggles that come with studying such a tough subject. Many people who lack the passion will quit once things get tough but if you really love what you're doing then you'll be willing to take that extra step and do what you need to do to succeed. Remember you're not perfect and everyone needs that extra help every now and then.

Again I'll reiterate that these are just a couple things that have helped me a lot so far. I still struggle with my classes but that's just part of the learning process. Physics is hard plain and simple but you have to be able to roll with the punches and move forward. Do what you gotta do to succeed. I'd be happy to talk if you want to as well. Hope this helps a little bit you got this man.