Hi...I have been going through it too...my husband lives 3 1/2 hours away during the week because of his job and comes home on weekends. On June 3 I drove up to his place and discovered he had another woman living with him! That explains why, in the month of May, he was only with me three nights. That explains why, in the last two months, he only called me from his car on the way to work. I had even come right out and asked him if he had a girlfriend or someone taking care of him or living with him, and he'd laughed it off like I was crazy. He had been sick all this time, and has passed it on to me, so I'm going through dealing with all of this betrayal while I am very sick, coughing, lungs hurting. We had just remortgaged my home to pay off his business debts (he'd had a failed business a few months prior to this).
I threw the OW out and made him promise NC, but he broke it...she didn't leave him alone and she stayed with him a couple more days, which I didn't find out until later. He wants to save our marriage and was finally seeing the light with the OW so he moved so she can't find him and he is currently looking for work in our home town area. OW has wreaked havoc on us, identity theft, stole his car keys, got his car towed, is sending me vile email, playing games. I reported her, changed pws and pins, we moved his car so it's in a secured area, etc. I told him to ignor her and we'll stay a step ahead of her, eventually she should quit. Meanwhile I'm going to take personal safety and learn how to use a gun for when I'm alone at home (she's intimated infiltrating my personal space). I think all of this has been a lesson to him.
In regards to "once a cheater, always a cheater", I don't think that's necessarily true. I have two BIL who did once and never again. I really think everyone's situation is different. Follow the instinct of your own heart as to whether or not you think you can give him a chance again. I am working to save our marriage but also have a "we'll see" stance as so much is in his court, and I need to be realistic and have my eyes open.
You mentioned having a BPD, please do not be so hard on yourself. There is no shame in having a condition, it's treatable, and you don't even know for sure if you do...you are NOT flawed in any way, regardless. You did NOT bring this on yourself or deserve it. As someone mentioned already, it is your husband's issues, not yours...just as it is my husband's issues, not mine. However, we are left having to deal with it because it affects us. You have supportive in-laws, that is good...I don't know my in-laws, my H' family is not close and don't have anything to do with each other. There isn't much you can do about rebuilding your M until your H decides to have NC with OW. As I've explained to my H, when there is contact, it brings us back to square 1 and prolongs the healing and rebuilding and I have no intention of continuing in that kind of environment. To decide if he wants me or a divorce...he's chose to stay with me and work on the marriage.
Rebuilding trust...it will never be the same again but our M can be better than it was before...how good is it if it is built on lies and disrespect? Right now we feel like we're on the same page at least and working towards common goals. Do I trust him? Hell, no! Will I ever? I don't know. I would like to but don't know that that can happen, again, I have a "we'll see" stance.
I am learning to take one day at a time, to not feel frantic about whether we'll make it or not, knowing that much is up to him. I have a willingness to work on it, but beyond that, I can't control the outcome. I feel much better than I did just 3 1/2 weeks ago. For me, one of the bigger issues is that he no longer lie to me. I would rather have the cold hard truth that I can deal with that a pack of lies I have to keep finding out the hard way. It is only a matter of respect.
Please be very, very kind to yourself, you have been through so much and you are so young, this has to have been tremendously hard on you, my thoughts are with you and I truly wish you the best.