The modal should involves things that can happen in the future. They are real possibilities, not just theoretical. Therefore the verb in the “if” clause should be in the present.
So the correct sentence is: [color=blue]“You shouldn’t eat so much if you want to feel good.” The idea is that the person probably wants to feel good, and if he doesn’t eat so much, he really will feel good. It can really happen.
If you change the modal, you could say: [color=blue]“You wouldn’t eat so much if you wanted to feel good.” In this case, you’ve got an outcome that probably won’t happen. The person doesn’t care about feeling good, so he probably won’t eat less. So none of this is going to happen. This is when we use would and put the “if” clause in the past tense.