I’m still fairly new to all of this so I didn’t know what fear of engulfment was, but after reading about it I can assure you I’ve never felt that from her.
If she experiences fear of engulfment, it wouldn't be with you. Yet.
Not until you were fully available and pursuing a real relationship. The appeal of an affair is that neither of you are available. The guardrails to real intimacy are in place. Remove a guardrail and that's when fear of engulfment would build.
I’mShe does make comments about how she just wants her husband to leave her alone sometimes and not smother her, so she may be feeling that from him.
This would likely happen to anyone occupying the role of primary relationship.
You cannot engulf her because you're the outsider.
You could very well be right about how I stabilize her marriage - certainly I give her things that her husband does not, like attention, compliments, excitement, etc
She may be getting these things from him. Many pwBPD (and many of us from dysfunctional families of origin) triangulate others in order to stabilize relationships. Telling you what she doesn't get from her husband sets you up to be the guy who rescues her. If you're codependent, you'll find it intoxicating to be her white knight. This dynamic is captured in the Karpmann drama triangle.
That’s probably what will eventually happen to be honest but it’s so hard to let go.
Given what you're shared here, it sounds like having a third leg on the stool (affair partner) is part of the way she copes with her BPD symptoms. Being with her probably also means accepting that there is a good chance she will set this dynamic up again. If she did leave her husband, which assumes you are getting an accurate portrayal of what actually goes on in her marriage.
I'll give you an example of how distorted reality can be for someone with BPD.
I have two adult stepdaughters. One is 28 (not BPD). One is 25 (BPD).
My husband and SD28 (not BPD) had a big argument when she moved in with us during grad school.
SD25 (BPD) was also living with us for the summer between semesters at college.
After the argument, H left the house to cool his jets. SD28 (not BPD) left to do the same.
SD25 (BPD) came out of her room and called her BPD mom. I was in my home office and she didn't realize I was there. She told her mom everything that happened between H and SD28 (not BPD) except in her retelling of the story, this all happened to SD25 (BPD).
Meaning, SD25 (BPD) either recognized that the argument was between her sister and her dad, and lied (to get sympathy). Or, she genuinely believed (felt) the argument happened to her.
Both are possible with someone suffering from BPD.
Everything that happened in our home was shared with BPD mom, and of the things I overheard, much of it was either not true or deeply distorted. SD25 lived in a different reality than the one the rest of us occupied in the home.
Some examples:
SD25 to BPD mom: They make me cook meals for them.
Us to SD25: If you want to learn how to cook let's show you how.
SD25 to BPD mom: They never include me.
Us to SD25: We're going out for dinner to celebrate our anniversary.
SD25 to BPD mom: Dad told me I'm immature.
H to SD25: Here are some things I think are helpful to know as you become an independent adult.
Does that make sense?