Respectfully, it does not seem like you want to CTB because you are not "smart". It seems as if you want to CTB because of failure in pursuing your academic dreams. As an old guy who failed numerous times, I can understand your desire to check out. However, have you explored your learning style? There are different types such as visual, auditory, kinesthetic, social, etc. I have come across "dumb" people but they are also "ignorant" of their lack of intelligence - think of the Dunning-Kruger effect. The 2nd part to DK that is often overlooked is people who are quite competent in certain areas feel less adequate because they are smart enough to be aware of their short-comings. This seems to be a fancy way of saying that if you are worried you are dumb, then by definition you are probalby not.
Now, regretting past failures and decisions are completely natural, but those are not tied to intelligence. Decisions and choices tends to be tied to lack of experience which leads to bad decisions. The bad decisions lead to experience when then leads to good decisions. I understand this seems hokey but it managed to keep my from offing myself due to academic failures.
And poor grades simply means that your answers do not align with what the professor expects. I bet if you found something you were passionate about your ability to retain and regurgitate information would improve. BTW, the ability to retain and regurgitate is no longer considered the bench mark for intelligence.
Look, I'm all for CTB ....... except when it comes to failure.....because after 60 years on this earth, I found that those who fail the most usually end up the most succesful. I've said in previous posts that success can be obtained simply by showing up and not quitting. But then again, I'm an old guy posting on a suicide discussion board - by definition, I could be considered a failure.