You don't.
You kinda just live like a ghost for a while till things settle down on it own.
Just continue doing whatever youre doing, the usually, til the waves of emotions hit then watch some cheesy movies or anything really (recommends things that can make you cry), eat tons ass of ice cream, drink tons of warm water later on so you don't have sore throat.
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And don't look at old photos, texts, etc, just don't. You can once you're over it.

Don't be afraid of pain, deal with it and let time fix it. Use the time to appreciate the good things from the relationship and learn from the bad.

I agree with both replies above, use each relationship as a lesson. Try not to regret but you can let yourself want what you had back, be sad, cry, curl up in a ball and just let it all out. I think time is really the only thing that helps make it feel less painful, eventually it will fade and eventually you will be able to see that person again or think of them with a soft remembrance, rather than a yearning or that really tight chest sting. I don't know if you feel this but sometimes when you're the one who broke it off, you feel guilt but please don't, if it is best for you in the long run then you did the right thing. You are also allowed to grief, even if you initiated it because you still have all those good memories. Keep doing what you do but talk about it when you need to, repetitively if you want, cry about it but also look forward to the future. You're going to grow from this, you're going to be a new and stronger person and it's going to be okay.

Google “mental grounding”. It’s a helpful technique when the waves of emotion that @rosalinas mentioned do hit.

Keep your routines going, do something physical that gets your blood pumping.

This all looks like pretty solid advice. It’s going to hurt. It can’t really not hurt if you care about a person and lose them. New pain is almost always worse than a pain you’ve felt before because you don’t know what to expect and you haven’t lived past it before to be able to tell yourself that you know you can make it through. Know you can though. Don’t try to forget. This was part of your life. Just know there’s so much more to your life and so much more life to live in front of you. Work in loving yourself and taking care of yourself right now. See the opportunity in front of you.

No relationship is perfect and not to be insensitive- you know this one wasn’t because it ended for a reason, even if that reason is just life taking you in different directions. This was an experience. You found things you like and likely things you didn’t in a partner. That’s good. You have memories and experiences and will make new ones in the future. It’s ok to be sad. That’s normal. Don’t turn away from your feelings. Acknowledge them, reconcile them as you can- but don’t let them control you.

Take time for yourself. Be with close friends and loved ones. Pamper yourself a bit. Do things you couldn’t do when you were together. Being single has advantages. If you can’t find anything to do- then it’s possible that it isn’t even the relationship that you miss so much as it is the hole it filled in your life. Self actualize as a person. Explore passions and hobbies, invest in yourself. Don’t use work or study or self improvement to avoid coming to terms with your feelings, but it can help you gain distance and perspective from overwhelming emotions. Keeping your mind occupied and allowing yourself windows to introspection while not dwelling on your feelings is crucial.

And just invest, invest, invest. It’s just you now. You are the thing you’ll always have until you die. You have to be ok, able to be happy, with you. You no longer have to accommodate the feelings and schedule and needs of your partner. Use that time for you and the other people in your life you love. Put your effort and energy into THOSE things most of all next to yourself. It just takes time.

Lives in general, but especially early and first relationships often stay with us for life. Not like it is now- intense and consuming, but most people look back once in awhile and feel little pains or think “what if” over past relationships, even people happily in love.

Hang in. I know it is tough. You just have to make it through day by day.