I'm planning to start dating someone who already has a partner, but I have some concerns

I am about to go into a relationship with a woman who already had a boyfriend. Usually I'm very territorial but I'm willing to make it work for her. Her other boyfriend is also quite territorial and the boundaries have been set by him. I am not very committal in relationships and his boundaries have benefited that side of me, however I'm worried his influence may have a negative effect on our relationship. How can I respect his wishes and also make sure I feel fulfilled in my relationship?

To be honest, I have never seen it work well when someone isn't really okay with polyamory but is "willing to make it work" for a specific person. It's like moving into a house with one horrible feature that you know you'll hate. You promise yourself that you'll "suck it up and ignore it" and then "get used to it" and that the low rent and sunny bedroom are worth the stove with only two burners in a micro-kitchen with no counter space. But over time it drives you nuts. It isn't really what you wanted. It will always feel like a frustrating compromise. Think hard about what you are committing your future self to. Be very, very intentional about this.

I cannot write you a specific plan to "make sure" you feel a certain way. I don't know you, I am not you, and even you can't ensure that everything goes smoothly. What I can do, though is strongly recommend that you do a lot of introspection about this. I can ask questions - your answers to those questions are actually the answer to the question you asked me.

This guy sounds like he has a lot of boundaries that you'll need to respect. Whatever you think has been laid out, know that this will create complications that you can't foresee right now. Do you have the patience, flexibility, and security in yourself to manage that?

You say that you're "very territorial" - what self-work have you done to make sure that whatever behaviors and feelings lead you to that conclusion are being managed in a healthy way? Are you working to get to a place where you're more okay with the polyamorous relationship, or just working to ignore the negative effects and territorial feelings? What are the "negative effects" you're worried about? Which ones are dealbreakers? Which ones do you plan to just muddle through? What's your plan for that?

Why are you deciding to do this? What is it about this woman that makes it worth it, to you? Does she seem willing to help you manage the newness and the fears going into this? How much do you expect this woman to take accountability for situations that negatively impact you and be willing to help resolve them? What will you do and feel if she doesn't meet those expectations? Has she been willing to compromise, or is she expecting that you get 100% on board with whatever it takes to keep her current partner happy?

Are you trying to ignore certain things? Hoping some issues will just go away? Is there any denial or willful ignorance going on? Anything you're hoping will change as time goes on? What's your timeline for that? Would you be okay in this relationship a year down the road if nothing has changed?

This is a time to be really honest with yourself and really intentional about the choices you're making. Don't just assume that things will work out because you want them to.