What signs might indicate that someone is slipping into narcissistic behavior, and how can they address it?

How do I stop having low self esteem and forgive myself for things I've done in the past? Because I feel so bad about who I used to be and to cope with that I punish myself and I don't allow myself to be happy. How do I get over it and move on?
Why do some people hold grudges when they're being stubborn and wasting their energy not working on themselves when they know nothing is going to happen, but why continue to hold a grudge? I'm flabbergasted and baffled.