I've been thinking about this on and off for the last few months after I asked some questions of an ex about their relationship. I think it can be really hard to tell where the line is between "difficult, but a worthwhile relationship" and "not worth it / possibly/definitely abusive." And this is not just for romantic relationships. For me the line can be found with these questions.
The basic questions I ask to determine if the relationship is abusive:
Can I trust this person not to lie to me?
Can I trust this person not to deliberately OR carelessly attack me (with words or otherwise)?
Can I trust this person not to touch me in any way I do not want?
Can I trust this person not to try to manipulate or force my behavior?
Can I say no to or disagree with this person with very little fear or guilt or worry?
And to determine if it is a worthwhile difficult relationship:
Does this person want to give to me at least as much as they want to get from me?
Can I share all parts of myself and feel respected, safe, accepted, and sometimes challenged by this person?
Does my time spent with this person make me feel nourished and fulfilled in the ways that are essential to me? (for me these are in meaningful sharing, in empathy, in cuddles, in curiosity, in openness, and in growth)
If you ever see me in a situation that looks like I am not asking myself these questions, I would love if you would call me on it. I realize important things in times of good and forget them in times of bad, so often.