End All Contact With Him Immediately
Unless you're willing and able to separate yourself emotionally from Jacob and continue having a physical relationship with him, you need to end all contact with him immediately. I do not make this recommendation lightly, as I have experienced to some degree the pull toward more than one woman at a time. However, I picked up on some statements you and he made in your communications that tell me this is bad news.
I Don't Buy His Story
Particularly disturbing is this whole business with the bra, panties, and "meant to send them back." Sorry, but I don't buy it. If the relationship with you was as important to him as he claims, Jacob should have either burned (or otherwise disposed of) the stuff as soon as he got it, or sent it back with an appropriate note to the woman in question IMMEDIATELY read: within 24 hours, if not sooner. I would have also probably shared the situation with my partner, had this happened to me.
People Move for More Than One Reason
The other thing that hit home was this business of moving a thousand miles to be with you. A young woman I was involved with a couple of years ago for about three months at a distance of THREE thousand miles moved to be with me. We were broken up within three or four months of that, and she was married to another man within less than 18 months of that.
Sometimes, people move for different reasons, and the relationship they're in at the moment just serves as a catalyst. I agreed to her move, among other reasons, so we wouldn't be left with a big "what-if?" in our lives, something you also refer to.
You Deserve Better; End It Now
It seems to me that the only "what if" left in this equation is "WHAT will be the magnitude of your pain IF you stay in this unhealthy relationship?" You said yourself that Jacob has poor judgment, low self esteem, and has betrayed you twice. You deserve better, and he needs to learn that those traits don't get rewarded. End it now.
Be Honest with Yourself and Others
You need to start being more honest with everyone around you, and with yourself first and foremost. You need to work through all kinds of issues, and you need to do this while not tying down someone else.
Your Needs Make You Dependent
Rather than truly being a "loving and feeling person" as you say, I think you're more of a person IN NEED of love and feelings from others, and have gone so far as to move significant distances to show that. You need first of all to find your own space and comfort zone, where you will not be dependent on someone else's validation to feel like a complete person.
You Would Have Acted Differently If You Really Love Her
If you really felt what you claim about Saibhin, I believe you would have disposed of the tell-tale bra and panties without any opportunity for her to find them. It will be very difficult, if not impossible, to convince any self-respecting woman whom you have betrayed to trust you enough to the point that she will re-enter a romantic relationship with you.
Find a Woman You Haven't Cheated On
The best thing here is to get to the point where YOU are certain that you'll never cheat on ANY woman again, find a woman who you haven't cheated on, and make her happy to be with you and you alone.
Perhaps you can then also be friends with women you have previously wronged.
I've Been There
Trust me, man, I am speaking here from experience.
All the best to both of you,
Saibhin's update of her story