If you're not feeling better by tomorrow. You should see a doctor. If you're not feeling better by tomorrow, you should see a doctor.
You're not feeling better by tomorrow, you should see a doctor. which sentence has correct punctuation

The second one is the write one

If you're not feeling better by tomorrow, you should see a doctor.√

If you're not feeling better by tomorrow, you should see a doctor. (first)

Explanation: the if is needed at the start

" I am tired of pretending that I'm fine to those around me, I've never felt more dead in my life." Believe it or not I can connect to this quote, there are so many moments in my life where I pretend I'm fine or happy when I'm actually not but I don't want to upset anyone so I don't say anything.