Why can't I get over her or him?

Try looking for someone new. Or take up a hobby. Maybe talk to a friend about it. Do something that will take your mind off of the situation.

It's possibly because the relationship built such a large emotion, you'll get over them, eventually.

It will take time, sometimes a lots, lots of time. Be patient. I know it's hard if not impossible to think you would ever get over, but it will get easier by time. If possible, try to distract yourself as much as possible - hang out with your friends, read/paint/write/draw something, try to remember something that makes you feel better and do that. With distraction and time you will get through it.

If we've been with someone for so long and invested so much into them it can be tricky to let go as we are worried what we will look like with out them, it's only natural

Do something purposefully to get your mind off whoever it is that you can't stop thinking about, like play a game, read a book, listen to music.

You convince yourself that no other man on the planet has the same qualities as him and thus, you have two choices: get him back or settle for someone who will never measure up. I hope you can recognize the absurdity in this! Will you meet someone else exactly like him? No, because no two people are exactly alike and even still, you and he broke up proving someone exactly like him is not exactly what you need. You won’t find someone with his exact qualities….you will find someone even better and more compatible with you.

You still hold the affections towards them because you shared many memories with them for a long period of time. The memories are part of you.

You can't get over him or her because you have yet to accept what happened. Once you do you will be free.

You've not successfully accepted the separation and refocused elsewhere. Emotions of him/her have not been completely assuaged for a true progression.

The friendships and relationships we cultivate over time are precious to us. Understandably, when we lose a connection that we feel is important, grief is a common experience.