My fiancé doesn't believe me when I'm telling the truth

[Kiwi girl]

Hi, I've been in this relationship now for 16months and 6 months ago he asked me to marry him and I said yes.
He has now found out that not long before I met him I had a brief affair with a married man, (something that I totally regret doing) the affair side was over well and truely before we met however the text messages (some of them quite explicit) carried on for 4months after we met because I cldnt say outright to this married man that it has to stop. As far as I was concerned the texts were just harmless banter.
My fiancé found these texts and is adamant that the affair was going on well into our relationship and to make it worse he knows the married man.
We have now been arguing badly for the last month because it took me 3 days of denying the affair before I fessed up and said yes I had the affair. I keep telling him that the texts are just banter but he won't believe me and accuses me of sleeping with him still and all my male friends (of which I do have a lot of) and accuses me of sleeping with people I meet in my job (I'm a tour guide so meet lots of people).
How can I get him to believe I'm telling him the truth and he's the only one I've been with and want to be with. He keeps saying that if I was telling him the truth there wld be 3 words that I wld be saying, those words I have no idea what they wld be, however I did overhear him saying to himself she has to say please so maybe that's one of those words.
I'm now so exhausted that it's affecting my work. I need some help and guidance in this. Thanks.

[PLEASE - NO RUN MESSAGES]

PLEASE - NO RUN MESSAGES This is a high level discussion board for solving ongoing, day-to-day relationship conflicts. Members may appear frustrated but they are here for constructive solutions to problems. This is not a place for relationship "stay" or "leave" discussions. Please read the specific guidelines for this group.

[Turkish]

As my therapist said to me,  "typically these things take a while to die down." It sounds like you were winding it down,  but verbal contact went into the time you were in a new r/s. It sounds like he's having trust issues.  I think in most relationships a new partner may act wary, though accusing you of sleeping with others seems over the top.  What leads you to believe he has BPD?

I wonder what the three words are... ."I love you?"

[Kiwi girl]

Everything I'm reading on here fits in with him, so that's what's making me believe that's what he has, also there is bi polar in the family.
I tell him every day that I love him sometimes 3or 4 times a day so that's not the words he's looking for.
He believes that because the texts were there then it's fact. No matter what I say or do he just won't believe me.
He's fine for 2 or 3 days then goes off on a rant and says he can't and won't get past it then turns around the next morning and says he can't live without me.
We do have something magical and it's been there since day one and is still there. So I believe that's it's worth saving even tho he says the most ugliest things about me. I try to ignore what he's saying as just a rant but it's beginning to wear me down both emotionally and physically, during the last mth I've lost 8kg just from not being able to eat and stressed out all the time with the big ups and downs that he's having, he says I'm not the person I told him I was and that everything about me is a lie, I'm a lying deceitful conniving untrustworthy slut and to go back to where I was and sleep and flirt with all my mates, and that's the person I am so no matter what I say when he's on a rant that's all he believes. I know I'm not that sort of person and I do have high morals etc and yes I'm ashamed about the affair but it can't be undone.
Where to from here?

[Turkish]

I'm sorry that he is using such abusive language with you.  It sounds like it's resulting in anxiety on your side (I can relate to the lack of appetite... .I was there once).

Though no one is responsible for how another feels or acts,  people can certainly trigger one another, in any relationship.  If he's BPD, then he likely has trouble regulating his emotions (the differences between his night and next day feelings) as well as probably suffering from a core wound of abandonment.  You can start understanding BPD and also learning tools to communicate in order to reduce triggers. These can be found on the lessons on the Improving Board: https://bpdfamily.com/message_board/index.php?board=6.0

I can imagine that his 3 word test is immensely frustrating... .

[Kiwi girl]

The words I'm taking as just words and are not meant.
The three word test is extremely frustrating.
I have read some of the posts on the board however I feel that I need to discuss/talk about. this now as it's at crisis point and don't know where or what to do, I need help now

[Kiwi girl]

Yes he does have abandonment issues, his father left when he was young and then he lost 6 of his best mates in a car accident not long after that, his family don't talk much to him because of his ranting and his children never visit.
I keep telling him I'm not leaving and most times walk away when he starts ranting but some times he follows me and keeps going on and on and over and over he same thing.