I feel you. I’ve been there myself. My own heart has been broken many times. There’s no way to prevent heartbreak; the risk of having your heart shattered is the cost you pay for intimacy. Here’s what I would suggest:

1. Grieve. Grief is a process. It’s okay to deal with heartbreak by spending some time curled up in bed with Netflix or writing bad poetry. Do what you need to do to take care of yourself.
2. When you’re ready, spend some time thinking about all your heartbreaks. Dig deep. Look at the people you were with. What qualities did they have in common? What attracted you to those people, and why? What signs of incompatibility, if any, did you ignore? A great many relationship problems can be stopped before they start by careful partner selection.
3. Look at the way you behaved in your relationships. Again, dig deep, even if it takes you face to face with things about yourself you don’t want to see. Do you do things that drive people away? Are you controlling? Jealous? Possessive? Can you stand on your own, or are you needy? How well do you communicate? How do you deal with conflict? Do you treat people well? Do you act with integrity?
4. Take what you learned in steps 2 and 3 and figure out how to do better in the future. How can you choose more compatible partners? How can you be a better partner? What values do you want reflected in your relationship? How do you embody those values? What qualities do you want in a partner? Do you yourself have those qualities?
5. Understand a very important and non-intuitive idea: Reacting to heartbreak by putting up walls will make it more likely that you will be hurt in the future. It took me decades to learn this! If you react to heartbreak by putting up walls to keep people away, decent people will see your walls and stay away. That’s what compassionate people do: they respect your boundary. The only people who will ever get close to you are abusers, because that’s what abusers do: they ignore boundaries. The more walls you put up, the more you discourage decent people and encourage abusers.
6. Move in the direction of greatest courage. Having your heart broken is the risk you take. You’ve had your heart broken already, so you know how devastating it is. You also know you can survive it. The courageous person says “wow, heartbreak is awful, but I can live through it and come out the other side a better person.” Love is for the brave.
7. Trust. You will never have a good relationship without trust. “I’m afraid of being hurt, so I won’t trust my partner” is pretty much a guaranteed recipe for being hurt. How do you learn to trust? By trusting. You learn to ride a bike by riding. You learn to knit by knitting. You learn to trust by trusting.

The clouds will part. The sun will rise again, I promise. It can be hard to believe when the darkness is all around you, but dawn is coming. You just need to find the courage to embrace it.