Is it bad that I still think about my ex even though we broke up 3 years ago and I'm in a long term relationship since?
Why do you still think of your ex?
I can't help it.
I understand feeling nostalgic.  But, you shouldn't hold on to your past like that.
I can't change the way I feel.
You are going to sabotage your current relationship if you can't get past your past one. You really need to find a way to move on.