have you washed it?  give it a good scrub with a nylon nail brush and dish washing soap, rinse in warm water then dry with a towel, you will probably need to leave it dry over night.  As for changing most people never change them for any other reason than vanity. But your should last a lot longer than 5 months.

Scrub some soap on it, and that'll get it tacky again.  I switch my grip every year.  I'll just pick up a new grip with a release, and keep it in the stable.  It's very easy to do.