It's ok to have issues
and it's definitely ok to feel overwhelmed when you are exploring them.
Maybe it's me, but I find it fascinating to uncover the reasons behind why I feel certain things and why I act certain ways.
I liked what you said about the possibility of having B as an excuse for hiding from your insecurities about other men. I'd say just take things one step at a time and maybe tackle that issue once you're in a more secure place with the P situation.
I completely understand the difference between intimacy and sex and how that can make us feel when are partners are doing either with another.
My GF loves her husband and has an online partner whom she loves. Aside from that, I haven't yet had to deal with her falling in love with someone new. I've only just, after a year or so, got to a place where I'm pretty comfortable with her having sex with other people. In our relationship, we are polysexual. We do not even have overnight stays... and so far, we haven't really wanted them. I know that if one of us falls in love with someone else and starts to want weekends away with other people, that will be a new kind of struggle.
So I commend you for your bravery in dealing with it!
As long as you guys continue to communicate with each other and be gentle with each other, I'm sure that things will get easier over time. Hopefully P is sympathetic to your feelings, if you bring them up. And there's nothing wrong with bringing them up. It's a good thing.
If you are open to the idea of the two of you loving others, not just being sexual with them, then the fear of it will improve over time. Making sure that you and P set aside time to be 'relationshippy' should help you to feel important to her, and her to you.
That being said - if you only want to be polysexual and just have extra sexual partners, the fear will dissipate, but you do have to make sure you're both on the same page.
For example, I know I am capable of loving more than one person - I have done before. Currently, I do not want to be in love with anyone else but my SO. I have a complicated life and need to keep things simple
I don't hold hands with my other partner, we don't cuddle much, we don't do overnight stays... for us, this helps keep it in the FWB realm, which suits us both. If I started going away with her for weekends, my girlfriend would be understandably scared; because it's not the framework we've agreed on.
What framework have you and P agreed on? Just outside sex, or outside sex including love / extra primary partners?