Resentment, or the strong and painful bitterness you feel when someone does something wrong to you, doesn’t have actual physical weight, but it feels very heavy and can last a long time. Forgiveness is one way to get rid of resentment.
Sometimes resentment lasts for years. It can be strong and hard to pull out, like an old, gnarled tree root. While resentment over being wronged can come from just one act against you, it also can get stronger over time, as in "your growing resentment might just come from the fact that the team captain always picks you last." It also comes from misunderstandings, like feeling resentment over a dirty look you thought was directed at you but really wasn't. It's usually best to root out resentment early.