Well it sounds like you miss this other person. If they're still alive I would probably try to talk to them, explain your feelings and and let them know you want them back.

If the person is no longer alive then you need to allow yourself to grieve. You can allow yourself to revisit past times in your mind. You can keep their memory alive in your mind.

If the person broke up with you and doesn't want to get back together then try to focus on their negative personality traits. Nobody is perfect and you can surely find things that you are glad you don't have to put up with anymore. Focus on those things. Worst case, focus on how badly they treated you and reflect on what it says about their character... Maybe that will make them look less appealing. My experience is that when someone mistreats me (or others) in any way, it creates a natural deep dislike for them. Maybe focus on that.

Life is rarely fully comfortable. You're almost always dealing with something. The key is to not focus on the things that make it worse.