How do you draw the line?

Hard and fast, rip it off like a band aide.

Block delete and leave it behind you, archive it under "lessons learned"

Where to draw the line?

At the point where you know that despite what you think you may feel, its not reciprocated and definitely not leading to anywhere healthy. Which should be clear in your situation, not only do you struggle to stay clean, he's mentally abusive, you've tried to off yourself AND he has no problem potentially passing on diseases to you.

Love and respect yourself enough to know you deserve better...

No matter how much you want it, he is not it and never will be.