Two years ago something happened and I'm still upset over it which makes me feel pretty dumb.
What kind of thoughts are you having about it?
I feel like such a fool for still holding such a grudge, I should bury my emotions and move on. But I can't.
Regardless of what happened in the past, you shouldn't dwell on it. The past is the past, but you can move on.  Help is available if you need it.
You don't know what it's like to be me or to feel like this. I should keep these feelings inside.
You should release the feelings in a positive way and then move on from it forever. No one should ever obsess over what happened in the past, or else they'll always have trouble with their future.