Why can't I get over the breakup?

My girlfriend and I have been separated for over three weeks. Still, I'm not over it yet, and I spend hours sitting in my head on the bed and thinking or grieving. I haven't had a good night's sleep for a while. I just had and still have the feeling that she is the right one.
I actually have so much to do, for school, etc. Instead, you just sat on the bed for an entire afternoon and was useless.
We haven't had any contact for a long time now and I just miss the exchange with her, we always told us what was going on in our life, so often thought of memories and now to do everything "alone" and not a bit of her to hear it really hurts.

What can I do? I don't want to run after her, but if I keep doing this it won't lead to anything. When will it get better? Everything in my rooms reminds me of her, be it the bed she was lying in when she was here, the television we watched Netflix on together or the clothes we bought together… I can't "give up" all of that.

Put everything about her or what reminds you strongly of her in a box and throw it away. Otherwise only distraction with friends and time helps. Mourn quietly for her, but try to focus on your life, even if it sounds easier than it is.

Get distracted and pursue hobbies. Forget her.

Meet friends, pursue your interests, do sports (physical exercise tires you and you can develop a good body feeling at the same time, become stronger and more resilient), seek company, express your grief from your heart - and after a few months you will surely become yourself feel better, in any case more stable.

If you want her back then tell her that you still hate feelings for her! If she refuses then you are probably better and do something.

It took me 1 whole year to overcome lovesickness… It was hell.

If you can somehow, distract yourself…