My girlfriend and I are in a very serious relationship, we have been for years and plan to get married. She introduced me to polyamory which was a new concept to me. I told her I’m open to trying it, but due to my mental health I had boundaries and things I’m not comfortable with her doing with her new partner. She agreed & so far its been going okay, with her being very attentive to my needs. But I was wondering, if she were to do something I’m not comfortable with, would that be cheating?

Only you can really make that call. I think “cheating” is defined as “violating the agreed-upon terms of a relationship.” If you feel like her violating those boundaries is cheating, then yes, it’s cheating. If it feels more nebulous to you - like she did something you’re not comfortable with, but it isn’t a complete betrayal, then maybe it’s not cheating. 

For instance, if I ask my boyfriend “hey, do you mind if we reschedule our dinner date because I just got invited to a really exciting party?” and he says “well actually, I would be really bummed out, I was very much looking forward to hanging out tonight,” then I have a decision to make based on the information I have. I could choose to go to the party, knowing full well it would upset him, and face the emotional consequences of doing so. Would that be cheating? It would certainly be doing something my boyfriend didn’t want me to and told me he would be sad if I did it, but is it violating the fundamental terms of our relationship? That would differ across couples, as it should - there is no immutable relationship law that determines what is and isn’t cheating.

In general, I think specific boundaries in polyamory like “if you do XYZ with your other partners, that counts as cheating” very very rarely work out. Emotions and desires don’t ever stay neatly in the boundaries we prescribe for them. Setting up instances in which you consider yourself “cheated on” puts you at a pretty high risk of feeling/being cheated on. My advice is to think through the boundaries you’ve set to try to understand where they’re coming from. Consider whether you can address your emotional needs and safety using strategies that don’t create such a clear-cut “cheating zone” for your partner to risk straying into. How can your partner help you feel secure in a ‘positive’ way (by actively doing certain things) rather than a ‘negative’ way (by not doing certain things)?

Also, if you know there are mental health issues at play, definitely discuss this with your mental health care provider. These kinds of boundaries can be useful for easing a nervous partner into a poly relationship, but I have rarely seen them work out in the long term. Seeing them as temporary tools to help you understand your needs and how to meet them, rather than new core terms of the relationship, would help a lot to alleviate your concerns about what counts as cheating and whether it’s happening.