One of my girlfriend's exes popped back up in her life and wants to date her. Exclusively. If that's what she wants I'd never stop her, but he's an addict that rides the sobriety line constantly. This factor has hurt her in the past and I don't want her to go through that pain again. Should I tell her I don't feel comfortable with her dating him because of his past? Or am I being controlling?
If he wants to date her “exclusively,” then she’d have to leave you for him, in which case, none of this would be your circus nor your monkeys. Of course you would “never stop her,” because it is impossible to stop someone from dating someone else, and it’s inappropriate to try.
It is not controlling to give advice, to remind her that he has hurt her in the past, and to encourage her to really think through what she wants and what choices are most likely to get here there. You can tell her that because you care about her, you’d recommend against this; that if you were her, you wouldn’t do it, etc. But people are going to make the decisions that they are going to make, and it’s ultimately not within your control, no matter how right, or persuasive, you are.
If I’ve misread your use of the term “exclusively” and the situation you’re being pitched is one in which she dates both of you, it’s within your rights to say “I’m not comfortable dating someone who is dating this guy,” in which case it would be on you to leave the relationship. You’re not threatening to leave her if she dates him, you’re not forcing her to choose - you’re making the best choice for you based on the situation you find yourself in.