Why am I so jealous?

I'm a single male in his mid 30s and never had a gf although I tend to get a few dates on dating apps nothing has been more than a hook up. A couple months ago i started hooking up with this woman from work although we got along well she was a bit "wild and free" so to speak, bit of a hippy, very open, vegan and had some interests that some would think is weird. At the time I didn't think our personalities matched but we ended up hooking up on a few occasions anyway and I really enjoyed the time together however I asked her what she was looking for out of this and she said nothing specific and was happy how things were. I told her I enjoyed the freedom of being single aswell mainly because I didn't think anything between us would and she always seemed to be busy so we didn't get many chances to catch up out of work. Since then we stopped hooking up and she's now started seeing another guy and for some reason I'm extremely jealous wishing I had kept my mouth shut and actually started dating her but I don't know why. It's not some mild jealousy either, I get extremely depressed about it and sad but I'm the one who thought it wouldn't work and didn't see our personalities working together. I'm not sure if it's because she was showing me a lot of attention which I usually don't get and she was interesting and now I have a sense of loneliness.

Any advice on how to get over this and insight on why I'm like this is welcome