Hi, I'd like to help you with your question.
It sounds like when your boyfriend broke up with you all of a sudden, it triggered something from your past about being abandoned. When he suddenly ended the relationship, that brought up all of those unresolved feelings and you were traumatized by them.
When you look at the relationship, there are two things going on here. One is your feelings about being abandoned. The other is the trust you have in him. Part of that is about your past issues but part of that is because he did leave all of a sudden and without explanation. In any relationship, no matter how secure the partners, suddenly leaving and ending the relationship is going to cause the trust in the relationship to dissolve. And in order to continue the relationship, the trust has to be rebuilt.
You cannot control what he does and that is the first thing to allow yourself to realize. You may want a guarantee which would be wonderful to have, but no relationship can offer that. There needs to be a balance between trust and independence. You have to assure yourself that even though a break up would be bad, you would get through it and be ok. You would not fall apart or be damaged beyond repair.
Also, letting go allows you to be your own person and for your boyfriend to be his own person. It is hard to let go because our nature tell us to hold on tight to something we want. Letting go sends danger signals. But this is how you hold onto a relationship. By allowing the other person to have choices. Those choices should be within the bounds of the relationship, but other than that, the other person need to be allowed to be themselves and offer their love as they need to.
It is not too late to repair your relationship. Seek counseling. That is the first step. The issues you are dealing with are deep seated and talking to a counselor will help you find the source and address it. Also, consider asking your boyfriend to go with you. Working on the trust in your relationship is important. He needs to explain why he left so suddenly and how you can trust he will not do it again.
Also, work on your issues through self help. Talk to your boyfriend about it. Don't make him emotionally involved, but just as a way of letting him know you are working on these issues and taking them seriously. Here are some resources to help you:
If you seek help and show your boyfriend you are working on your issues on your own, he will see by your actions that you are serious about the relationship.
I hope this has helped you,