Sounds like Tom might’ve had regret and couldn’t maintain a happy relationship with you. It does sound likes the drugs took a toll and guided him down this path but if he doesn’t own up to his action he’s still in denial. Either way I’d personally drop him as you sound like a sweet heart and you don’t deserve that. Also get tested if she uses needles she may have hiv and could’ve passed it to him and him to you. I wouldn’t get back with him solely on the fact of having sex with her, you could’ve caught something.