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Don't get back together with the ex. If he ever actually gets in to medical school, he'll then be surrounded by all these new med students who he thinks are brilliant doing the right things with their lives because its the same path he's chosen, and he'll be back to looking down on you because you're not one.
And don't be so sad about the way things ended with your latest boyfriend. That was the best possible way really, its so much worse to have to dump someone especially because you're getting together with someone else, and leaving him with no chance to work things out if thats what he wants to try. This way he's left, there's no messy horribleness coming to worry about.
In short, change your attitude. You're creating your own misery.