Kinda maybe have a crush on a coworker, but I am completely emotionally messed up and lonely due to the death of my boyfriend. What to do?

This is probably not an average question, so please bare with the details.

I'm 21, I was dating a guy I truly loved and wanted to marry for three long years. He was my biggest support through college and I don't realy know how to carry on happily without him. He died of cancer at the end of September - although we didn't know about it until the last second, despite being in the hospital for the symptoms. Who even gets gastric cancer at 29 anyways? So despite me somehow carrying on in life and making good grades still, I feel like my personality has greatly changed and that I find myself being riskier and more careless than I used to be. Basically, my common sense feels worn down by everything that has happened (along with my health and well being, among other things).

So now that the explanation is out of the way, this is the problem: I work in an office, as an engineer. When I started early in the summer, I had one of those lite crushes on someone (he's really cute and likes metal music like me). Of course I kept that to myself because I was more than happy with my boyfriend, plus this guy had a girlfriend at the time. The company had this big party at the end of the summer at a hotel, and we were allowed to bring one guest. I brought my mom because my boyfriend at the time wasn't really able to eat anything (we thought he had ulcers). This coworker was alone, newly single, and showed interest in me. He asked me why I didn't bring my boyfriend and I told him it was (cont)