tl;dr: Got fixated on a potential romantic interest, they're not single and I feel horrible, what do I do?

Hi all,

I found out yesterday that someone I really really like has a girlfriend of 4 years. Honestly, I have "known" them for about 6 weeks but have seen them for a total of 9 days and maybe a combined total of about 24-30 hours. I feel so stupid. I got insanely infatuated so quickly and intensely, and did a creepy level of social media stalking trying to figure out if they had a girlfriend before I committed to liking them, and I thought I had figured it out that they didn't, so I let myself feel my feelings, privately, but yesterday really crushed me. I was out with friends when I found out and I was ok letting them know how upset I was, but after I dropped the last one off, I basically bawled my eyes out the whole 20 minute drive home, and couldn't sleep afterwards. I'm exhausted and in such a depressed fog, the only thing I can focus on right now is this, and maybe the worst part is that I have to see him later today. Thankfully I don't think I've been too obvious about my feelings except to close friends who are all really good at keeping secrets, but honestly I'm kind of terrified. I feel like I might break down crying or not be able to engage in the same way, and it's not that likely that he'll ask what's wrong, but I absolutely cannot be honest with him about this. My last relationship ended basically in flames and this is the first time since that I've allowed myself to really get into romantic feelings, but now I feel like it was a huge mistake to do so. 

I've had this happen before and I talk about it with my therapist, so I feel like I'm pretty good at recognizing when it's happening but I have no idea what to do to actually mitigate those super intense feelings. She's told me to focus on other things like work and school and hobbies but honestly the feelings completely take over my life at times, and it's basically impossible to remove myself from them through stimulation alone, like I need something that I can write down and stick in my wallet to read every day so that I can be more careful the next time and not be so "all-in" with my feelings and my energy. 

I know this is really angsty and emotional but I don't see my therapist until tomorrow and I feel so lost and detached, any words of advice or encouragement would be a blessing