Hi Lay, welcome

so glad the site has been helpful to you and that you've found valuable info here for your journey.
You've got a lot of moving parts going on right now, so just a few questions to help us understand your situation a little better --
How long have you and your fiancé been engaged and/or together?
When did he go to jail? If you feel comfortable answering (ok either way), what is he in for? I'm assuming that you have some kind of real-time communication with him -- is that by phone, or visit, or...?
You're not alone in having a loved one with BPD who seems to have some paranoia. His worries about your fidelity are showing up in a big way, and one of the things that some members here have learned is that trying to "prove" your faithfulness, or "prove" anything, really, tends to add fuel to the fire, even though it'd help a normal person. A pwBPD might experience the repeated "proofs" not as "ahhh, now I can relax, I finally know my loved one is faithful", but as: "the more my loved one explains why she is faithful, the more I become suspicious, because why would she have to work so hard to prove she loves me, unless she really didn't?"
Normal strategies to reassure a loved one sometimes don't work with a pwBPD -- and I think I hear you bringing that up when you say that it's hard to not JADE in practice. Typically, if we just Explained ourselves to a loved one, that'd help, but not with BPD.
As you work to avoid Justifying, Arguing, Defending, and Explaining, maybe you can look to substitute the JADE with SET? Have you heard of the
Support, Empathy, Truth approach yet? Could be worth looking into.
Fill us in on how you're doing, whenever works best for you;
kells76