How can I stop thinking about him/her? Why my heart hurts so much?

I have dealt with this same problem. It is hard to get your mind off of someone, but my recommendations are that you should try to distract yourself from that person. If it is a person you used to love, you can find a fictional character you really like and distract yourself with that character. It takes time to get over a person, but distraction helps a lot.

Give you some time to accept your separation... It hurt because you need time to accept this ending, for a new beginning

Breakups hurt very bad. Everyone goes through one. I know it can be very hard to "get over" the person. Taking time and doing hobbies, or your favorite things for yourself helps.

it is perfectly normal to feel like this. i myself are going through a similar situation. i find surrounding myself with friends and family. there is no length of time in which things like this can go away. take things at your own pace, make sure you talk to people, an just remeber there is someone out there for you.

Focus on yourself. Have you ever wanted to try a new hobby or just something new in general? I've found by getting yourself into something new helps your mind stop from being stuck on that one person. Now I'm not saying it's a magic cure but it certainly helps.

Heartbreak is one of the hardest things you can go through. Missing someone is very natural, especially if it is someone close who we were intimate with, and stopping those thoughts completely may not be possible. Practising self-compassion can help to ease some of the heartache. Or finding other loved ones to spend time with. Don’t forget to continue taking care of yourself by eating well, exercising and sleeping at regular times. You might not feel like doing these things, but do try. It will help you feel better.

Moving on is hard. Forgetting someone you loved is even harder. I understand the pain. But no point dwelling in it. Life's worth living and finding new ways to enjoy it. Think of it as a chapter which ended. You're free to open up a new one.

Stop thinking about him by diverting your attention into other activities that you like,, set some purpose to achieve and forgive him not because he deserves it but because this will help you to move on. Remember whatever happens always happens for the best and it means he wasn't worth you, you didn't deserve a person who leaves you in the Midway but someone who will support you despite knowing all your flaws,, change always takes time but trust me it will be beautiful in the end :')

start loving youself in the best possible way you could 1 this is the best way to get over heart breaks

It's super natural that you feel hurt, try doing things to keep your mind occupied. Hobbies, friends, new challenges. Anything to keep you busy and learn some cool new things to learn that you can carry on, be awesome and maintain happiness!