What to do if i have a "not too bad" cavity. is there a way i can reduce it?

Usually no: Only the very, very early cavities, ones that are at the surface of the enamel, can be remineralized with special pastes. The rest of them need to be restored with a filling. Sounds like yours needs a filling, but only your own dentist knows for sure.

Get it filled: One a part of the tooth decays, it will not normally "remineralize". There are experimental techniques such as ozone therapy which purport to reduce the size of a caries lesion [cavity] but they are not commonplace recognized by the dental profession.