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I never thought that my ex truly trusted me. It seemed that she was afraid of me in some ways or thought I was trying to take advantage of her. She was also obsessed with domestic abuse and rape in either the news or drama. She would  interpret several of my actions as negative. Ironically between the two of us she seemed more likely to lash out physically than me.

Mine was very similar

Is it becoming clear why borderlines behave the way they do?

Yes in a weird way but they aren't held accountable for what they say and do

Did you hold your ex accountable statsattack?  What was your ex's reaction?

My uBPDw told me that she never really trusted me. Never mind I gave to her until it hurt, worshipped her and would do anything, I mean anything, for her. I never gave her a reason not to trust me.

I will say that once she found out that I had confided in a few close friends about our relationship when I was at the depths of despair, she deemed me disloyal. And I have never been right in her eyes since. So, yeah, that gave her "fodder" to support her feeling that I was untrustworthy.

Acted like a 5 year old. Couldn't handle having a 5 minute conversation

Sadly this is precisely why she didn't trust you; the closer you get the more they fear engulfment, so they push you away-if you don't go away and continue to try and get closer they ramp up their efforts to hurt you so you go away, very common is not trusting you to the point of believing you are unfaithful, then comes cheating on their part and projecting it onto you.

Interesting reading this.
So they don't trust you at all?

Mine is split.
We are divorced, I have access to all of her accounts, all of her passwords. Keycode to her house. I am keeping all of her important documents for her. She has stuff stored at my place yet. I work on her vehicle for her.

But if I was to tell her she has the most beautiful eyes she will not believe me. Or If I told her I never cheated on her she will not believe me.
So kind of where friend/relationship boundary is, that is where she stops trusting me.

Kind of weird I thought.

Mine told me we were better as friends. Kind of sounds familiar, but messed up at the same time.

Trust isn't black and white.  It's not an object.  It's a feeling.

BPD have issues with all of these.  We ourselves have issues with them, but maybe have an easier time managing them?

I was told the exact same thing. Even after she filed she says see we are better friends already.

Then she says nothing has really changed. I had to point out that everything has changed. I think it shocked her. She totally changed her plans and the way she treated me after that.

They trust you when you aren't too close, once you get too close you trigger their fear of engulfment-they fear you are going to hurt them (leading to abandonment) so naturally they aren't going to trust you. All depends on their reality based on their emotion of the moment whether they trust you or not, sad thing is they trust you less the closer you get-exact opposite of normal human nature.

The reason they commonly want to just be friends is because you aren't too close (they can't articulate why they do what they do and feel they way they feel, it's just feelings to them). The "friends" thing quickly changes though with their feelings of the moment.

is it possible for them to fake a relationship? Not be in with someone they love but someone who they can control and won't leave?

I don't believe they can "fake" a relationship, after all it is what they want more than anything. The "borderline" between engulfment and abandonment is always moving depending on their emotion of the moment.
BPD exists to deny itself, and the disorder always runs it's course.