I have a rash I my armpit that isn't going away, what can I use to treat it or at least treat the pain and itch?

It depends on the. Cause of the rash. If it looks like a fungal dermatitis you can use an over antifungal cream. If its an allergic rash the treatment will be different. There are multiple other causes and therefore you should see a doctor.
Rash in armpits. This can be a yeast infection or allergy to your deodorant-antiperspirant. If stopping the latter doesn't help, then consider the yeast infection. Sarna lotion is available otc and can help the itch.