listen, lady, #notallboys inspire feelings of intense attraction. Before you go judging us all on the unease caused by someone's charm and good lucks, remember that a lot of us cause entirely different sensations of unease with many fewer means at our disposal!
In all seriousness, the thing that helps me is consciously separating the person you know from the fantasy in your head. When I find myself thinking about some near-stranger and the lifetime we're [not] going to spend together, I start doing mental back-flips and tight-walks until they are not the person I'm fantasizing about. I wish I could explain what my thought process is that gets me there, but I hope this helps. I divert my flights of fancy into an introspective process where I consider what my expectations say about myself. Once I start to understand what it is I'm putting on them to fulfill, it kind of humanizes them for me. At this point fantasy and reality begin to separate again, and I get a little more grounded.
I understand that the main problem is that it's bringing up feelings you haven't had to deal with in a while, but the nice thing about separating out the fantasy is that it lets you indulge in the excitement without relying on the person who encouraged them, and without expecting anything from it. You've dealt with those feelings of loneliness before. You are under control. None of that has changed. It's just that your situation has led to a few things resurfacing.
Before you met this dude, you were having a great time. You were being fulfilled by a whole bunch of other things. Then you got some tingly feelings and it feels like you're missing out on something. Maybe you are, maybe you aren't. You're not going to know if you don't try it out, but if you don't, you're not really missing anything. Because right now, that guy is nothing more than your hormones and imagination. You're stuck with your feelings, at least for a little bit, but that doesn't mean you can't find a way to enjoy them healthily.
Sorry, that might not help a whole lot, but I've only recently gotten to a place where I can manage those infatuations, myself, so I don't quite know how to share helpfully.