Hi Che.
I don't think so much the result of your relationship with this guy was a result of sex as much as it was his personality failings. I have seen within my own family what cocaine addiction can do, and I'm aware of what heroin can do. I don't think you're to blame so much as that poison and what it did/does/can do to him. I do at least think that is a very significant facet.
I wouldn't advise you try to completely replace sexuality Che. Completely denying yourself of it isn't a healthy response. It's a matter of moderation.

You don't need so much to cut it out as much as it sounds like you need to cut it down. Doing it (or anything else) "all day" is a little much.

I'd simply advise you to cut it down, and I would think you'd be alright.
I do agree that "sexin' randoms" being retired is a good thing. I think you'd find a greater degree of stability in staying with one partner rather than randoms. I do think you're wise to acknowledge that the caliber of people such as your buddy you risk becoming involved with is lacking, and I think you're doing yourself a favor by cutting that particular aspect out.
To answer the question at hand though, a hobby should be based around something you enjoy. With a little more information as to what that may be, I can offer more detailed suggestions.

A lot of people suggest some type of creative activity, such as some sort of craft or writing. Personally, I go shooting as often as ammo costs will let me and I'm a collector of various odds and ends (fossils, coins...guns I suppose if you took an inventory

). It all depends on what you like.
I do hope I was of some help. I wish you my very best, and the best of luck.
My best,
Harley