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I admit, I'm not that much older than the LW, but boy, she does sound young!
LW, of course you're sad. You were dumped. It's normal. It does not, under any circumstances, mean that your relationship with your former live-in boyfriend was going anywhere. I mean, just read your letter. You two were miserable together.
You admit that the attention from your just returned from China going to med school ex, especially his regret for treating you like crap, was heady. Now admit that you used your med school ex as a way to break up with your live-in ex, without having to admit that you wanted to break up with him. That you're now ducking phone calls from the med school ex should tell you that.
You're starting grad school soon. Now is the perfect time to take a step back and just figure yourself out, before you plunge into a relationship with either of your exes, or even with someone new. Figure out what you want, from yourself, from your life, and from your romantic partners. Figure out how to be alone.