In January, you posted that your daughter, a senior, was way too involved with this boy who is 2 years younger than she is. You were quite convinced that it wasn't a good situation.
Now, your daughter has graduated and her boyfriend is going back to his high school for his junior year and she is doing something else. This is a good thing! She's moved on, and he seems to know it. Sure, it's hard for her to make this transition into adulthood, and it's hard for her to see her boyfriend hurt (if he is). It may well be that she's a little scared to move beyond the comfort of a super-easy relationship (with him much younger, less mature, and quite smitten) to go out into the unknown world of meeting new people and having new experiences. Maybe she thought she'd be the one to make the decision, but this time he beat her to it? She'll get through it.
What concerns me is how incredibly involved you are, crying and sobbing yourself. That's not healthy for her, or for you. You must take an adult step back here, and be more mature and less emotional yourself. You seem to feel that this temporary hurt she's going through is some sort of life sentence. It's not. Let her cry and be done with it. This was not your relationship, first of all, so you have to stop mourning this as if it is a death. You have to stop viewing your adult daughter as someone unable to manage her feelings without you getting down in the depths with her like it's the end of the world. She is going to second-guess herself if you don't change your viewpoint.
Your job as a parent is to convince her that she is a strong and competent woman who can take care of herself, and that her mission is not to make others happy (her ex-boyfriend, or you for that matter) but to make her own choices for her future. Her whole life is in front of her, and she's to be commended to making a painful step to move beyond adolescent safe relationships and grow as a human being. She will need this skill to manage in college or in the work force without always being able to please everybody else (professors, roommates, bosses, etc.). Be proud of her. She needs a backbone - and that means you have to demonstrate that you have one too.