Being able to "let go" (of resentment, of jealousy, of hate, of bad memories, of past relationships, of criticism, of negativity in general) shows maturity. You can be a middle aged adult chronologically, and yet still have the childish tendency to hold on to the bad in your past. That kind of behavior demonstrates insecurity in yourself and, indeed, a lack of maturity.
Stop. It's only holding YOU back! The person you harbor resentment toward? They have long forgotten about you and have gone on about life care-free and happy. The only person a grudge hurts is the one who holds it. Trust me - no one else cares. And the more "ugly" you perpetuate, the further you isolate yourself into a dark place.
Push away the negative and you will be able to grow as a person. Hold on to it, and it will always pull you back.