you sound exactly like me and my ex, up to the proposal anyway.
he started uni in the september, and he couldn't be without me the first week. i stayed over and stuff and i really thought it would work out. just after that week, he turns round and says 'it's not going to work' and i was like wtf!? it toally blew me away because we were so close. i was, as you, scared to lose him to someone else so i came across as very clingy.
still, we continued to 'see' each other up until january of the following year (5 months after he told me he didn't want to be with me) at which point, after many pleas of love, i told him to stop contacting me.
as you say, now, looking at the pictures, and seeing pictures of him, it's like i know him but i've only ever met him in a dream. it's like it all never happened eventhough i have all these photos and messages and emails to prove different.
it was hard in the initial months after the break up. i didn't want to let him go but in the end i had to, and it was horrible. but time heals all. i haven't had any contact with him now for over a year. i just see it as a chapter in my life where i learnt a lot and grew a lot.
he's turned into a bit of a wierdo now anyway so i'm kind of glad
plus i am living with somone a trillion times better who i love a whole lot more.
it's probably best that you move on. listen to yourself; he won't come back, and if he does, just tell him to get on his bike and bugger off! you will find someone better and you will be a thousand times happier. trust me!