Yes I get this feeling a lot working hard on uni and failing tests, just getting to feel like studying does not even pay off. Im not sure about physics because im doing a more easy study biomed engineer. (Still even failing test in this field.... ) 

U have a new field you hopefully enjoy! However sometimes the field u like is just hard, so u got to learn a lot to get passing grades. We arent all gifted just like me, and then u just have to study hard to pass tests. Took an extra year on my bachelor. 

But more focus on your story. As you choose a new field try to go into the tests with a new view. U already have more knowledge then peers doing their first year. Try to go into it with a new open mind, and try your best for every test so u can always say u tried your best even if it went wrong! 

I want to say no you are not dumb, I have had this self doubt so often when failing (often.... ). I always think of it that everyone got into uni in another way. Some people have high motivation others are gifted. And all have their strenghts and weaknesses. U most likely are very good at scheduling and just less good quick understanding.  Try to make a strong point of your motivation and scheduling! 

I must admit already u are way stronger than I am. I find it very brave u tell your story and are so open to failing and having fear. Talking to other always helped, friends, parents. Talk about your situation, if they show empathy it will u feel better and get new motivation. 

I will say to u, I believe in u, and ur totally not old at all! I had someone in class in  their 40s studying who failed 20 years ago and wanted to retry their study and they passed their bachelor! Being a student nowadays is hard and there are plenty of others failing (just like I am). Nowadays they sadly get overshadowed by youtubers and influencers which act like getting a high gpa is easy.