stealing, lying boyfriend

okay.
so, i've been together with this guy a little over a year now.
the entire time has been lying to me. he does drugs behind my back.
(even though he's getting 'better' for the who-knows-how-many-ith-time now...)
all my friends say he treats me horribly.
and now after a few days (i knew about it) i got him to confess that he stole
$20 from me. he's paying me back very soon. hopefully.
this guy has drug me through hell and back.
not to mention, i'm always the one that crawls back to him.
hes doesn't know what he wants half the time. sometimes
he wants a relationship, and sometimes he doesn't.
when hes in bad moods (he's very depressed) and i ask
him if he loves me, he says he doesn't love anything, including me.

why do i stay in this relationship? i don't know.
i feel so stupid for being with him, yet so lost without him.
during our many 'breaks' i always find myself caving in.

hes an amazing person under all this, he really is. hes a total sweetheart.
i know i cant change him, but i want to help him. he would be a great
person if he only knew what GOOD qualities he has.

what do i do? i'm totally lost.
and i KNOW i'm very stupid for staying with him. :/
but i just want to see him happy...