Good morning, Amanda.
Wow....I am so incredibly sorry this happened. The interesting part about being with someone who is unfaithful is that it will forever change the way you approach relationships. You will always be fearful initially, always struggling to trust. For him, there are no long term affects. You truly are the victim in this situation.
I realize he says he will never return and maybe that is the case but he is missing the point entirely. Yes, Thailand has many women who are looking to be with wealthy, successful UK men but there are also women like this all over the world. He simply turns on his charm and can be with whoever he chooses.
I can understand why you feel humiliated and heartbroken. I honestly don't know how you are functioning.
I can't tell you what you should do---only you know this but I do suggest following your instinct as it never lies...it's your truth. Should you decide to stay with him, I would highly recommend he seek psychotherapy to work through his ego/narcissism. Perhaps down the line, the two of you can attend couples work to start the healing process. This is a long journey, sweet friend, so please choose carefully.
We also deserve to be loved--and loved deeply. A person who truly knows and understand love would not treat another individual like this--especially a woman he desires marriage with. A solid relationship is based upon trust.
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Please take good care of yourself and feel free to contact me with any questions you may have.