How can someone make you happier than ever before, yet that same person make you the most depressed?

My best friend is just that... he's always there for me. But, over our time as friends I've come to find out that I really like him, and I've told him that, but at the same time he makes excuses for not going out with me, though he says he likes me too. And then he goes on about all these other girls that are adorable and he's "in love with", then will quickly reply "but not as much as you!"... but i still get so jealous, and ive cried myself to sleep over this countless times...

Also, he will keep so many details from me anymore. He'll start to talk about something and someone, but when i ask who or what he just answers "a friend/place" but then when i dont give him the details, he gets angry and frustrated, thinking i should tell him everything. What should I do?

And please, dont tell me to just get over him and move on because I have tried that and I just can't seem to pull myself completely from him because it would be losing a best friend, the one person who will listen to me no matter what. Also, I am 18, so this isnt just some love sick puppy thing, this is an actual friendship that seems to be coming apart at the seams, breaking myself with it.