How about instead of fearing feeling replaced, you spend energy cultivating the opposite? Do behaviors that you need to do in order to feel intimate, loving, close? You have a condition to deal with (PE) but does that mean you cannot share loving gestures? Loving talk? Loving feelings?
Could focus on what you want rather than what you do not want.
You could also say to your partner...
"Hon, I feel like opening our relationship is something I _could_ do, but at this point don't feel strong enough either with us, or with myself to do it. Could we discuss how to solve these other issues first?
I'm scared that I won't be able to digest feeling of inadequacy, both with you, or in contemplating pursuing other relationships for myself.
I need support in this area from you. I would like you to__(what?__
- Read things online with me?
- Reassure me?
- Create agreements and boundaries we both can feel good about?
- How we plan to handle conflict resolution when unforseen happens?
Could you be willing to help me in that way? For the next (Month? Three mos? Longer?) before we revisit the question if we are on better footing now to Open Strong and not Open Wonky?
You guys could read "Are You In Poly Hell?"
together and talk about dealing with feeling displaced, demoted, intruded upon, etc.
You guys could read about jealousy:
Making Peace with Jealousy in Polyamorous Relationships
(esp page 5 and 6 things)
Unmasking the Green Eyed Monster
Here's more to read:
Could demystify it so the fear lessens by learning more and forming how YOUR agreements would be like and what open model
you are seeking together.