My best guy friend has a girlfriend and im jealous/depressed?

So if you havent read my other questions, basically my best guy friend went out 2 yrs ago and we broke up after like a month of dating and i was basically over him until we fooled around a couple weeks ago. I knew nothing would come out of it but it still brought up some of my old feelings for him. So when i heard he got a girlfriend yesterday, i got really depressed and jealous. My question is, why? I know hes had feelings hidden for this girl for a while so i think they might last a while which makes me even more sad. I told him im really happy for him, which he seemed suprised at for some reason, which i am its just half of me is rlly upset about it too. Whys this happening? Why can't this feeling just go away. It's making me really depressed. Help?? What should i do??