You spend all that time being pissed off with someone so you're not going to just "let go" that easily, sometimes we're just too stubborn to let something go....  Is moving past the anger a show of weakness or strength?  My answer is strength but try telling that to the stubborn gene inside of me!  

Why hold your hand on a hot stove?" You know it hurts so why keep doing it?  

Some things are what we would class as unforgivable eg; sleeping with your best friend's girl/guy but why get hurt twice?  I'm not saying to be friends with them again or even forgive them for that matter but holding a grudge is only going to slow you down.  And pfft what would they care anyways, they stole your partner in the first place so I highly doubt they care what you think!!  

At the ridiculous end of the grudge scale you have your grudges that travel through countless generations, eg; He stole some hay from my farm back in 1788 and now no one in the family is allowed to have anything to do with their family...  200 plus years later and the feud is still going strong!!  I have seen Oprah programs where people forgive murderers for gods sake.  

So let's weigh up the grudge and the chain reaction that follows - initial event that pissed you off - anger, revenge, avoidance....  None of which is productive, anger never results in a smile, only cries.  So do yourself a favour and refocus all of that energy you put into being pissed off into something positive for yourself :)