Like most volatile type relationships, they were still messing around even though they never really got back together. One of those "they love each other so much they can't stay away, but neither is willing to do what it takes to make the relationship really work" type of things. So this was going on until Kate went crazy for Dax Shepard. Its killing Owen because Kate is obviously head over heels for Dax. She is even putting Dax ahead of her own son, which she wouldn't do for Owen (remember she insisted the relationship be kept quiet for her son's sake). She's even whispering to friends that Dax is "the one" and she might marry him f he asked and it got back to Owen. She stopped taking Owen's calls and no more booty calls, so Owen went into a pretty bad depression.
Contrary to popular opinion, Owen is as moody and dark as his brother Luke, he's just not dramatic with it. He just gets very quiet and withdrawn when he's low and is not an asshole about it. Also contrary to opinion, Owen is not a doper. He'll take a toke or snort a line to be sociable, but he's not a regular user and he does not offer it in his home when he has parties and stuff. So the whole drug thing isn't really true. He did take some pills and mix it with alcohol (maybe more stuff, not sure) to try to "go to sleep", which is what he told his brother Andrew. My contact says as far as she knows it was just pills, but its possible that he may have slit his wrists as well, but she doesn't believe he did.
So in summary of what I've been told, Owen went into a depression over losing Kate for real, even though its his fault as well. His suicide attempt was more of an immature "you won't come back to me so I'm going to kill myself" thing. Not that attempted suicide for that reason is not serious, but the loss of the relationship is more than half his fault and Kate probably shouldn't be totally blamed for it.
Oh yeah, as an addition. Luke really did have a thing for Kate as well, though nothing happened between him and Kate. Just some sibling rivalry BS really.