The only thing I can think of that you can do is start the ground work of your boundaries. Start talking about what you can accept and what you can't. Ask her to do the same until you reach a common ground. You might be pushed to compromise, she might be also, that's just how it is. Keep moving and adjusting the line of your boundary until it is mostly comfortable for both of you.
When someone comes along that is serious for her, the boundary moves again. It should be as fluid as you all feel comfortable. The most important thing is to keep being honest with each other and yourself, keep being open to new ideas and consider other peoples feelings as much as you can.
If she is good at poly in terms of how the two of you define it, then you should trust she will stick to what you have negotiated. If she is unable to, then its time to talk again. If she is disrespectful towards the process and towards your feelings, then I think I would look at whether it is worth staying.
Stubborn and disrespect are different things. Stubborn means she is sure of what she wants. Disrepect would mean she can't see past that in order to meet the needs of another without compromising her own needs and wants.
Hope this helps. Its my opinion from what I have practiced. It has worked for me to the best of its ability. Not perfection, but a start.
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