Look - there was some commonality between you and your then friend, right?

Usually friends have something in common and other things quite different, and that interplay of commonalities and differences is what sustains the friendship.

Given that there were commonalities between you and your then best friend, it is not altogether startling that your ex gf would eventually get interested in your friend. Add to this an extreme level of income disparity between you and him (presently) and that might have tipped the scales in his favor, and I do not mean just $$$ he spends on gifts he buys for her - he must be confident, comfortable, self-assured from all that wealth (which he got through his projects and not from his parents, so he is entitled to take pride in being financially independent). And you paled in comparison with him. Or maybe you did not pale in comparison, but simply your relationship with your ex girlfriend had run its natural course and had come to a halt.

These two people did not owe you the duty of care - there were not trustees obligated to protect your best interest because they were administering a trust whose beneficiary you were. I am rushing now and I did not have time to read through all the exchanges, but I think I noticed that Rose was telling you that your relationship partners have their own needs, too - they are not there to protect your best interest only. From what you wrote, the gf was caring when she was in a relationship with you, but eventually she shifted her interest on your friend.

The very best approach for you is to not erase them from your life but assume a cool attitude towards the whole thing and reconnect with them. You said that you pretty much did not have anybody else, so go back to them as a friend. Your former best friend possibly feels guilty enough to at least think of you first when some income opportunities arise on his horizon (unless he is independently rich and does not do projects anymore, something will come up that might be of value to you financially - people who do projects have needs or come in touch with others who have needs, sooner or later).

The imagination that runs those scenes of their having sex in your mind will eventually slow down, and if you are up to a little challenge, you can try the paradoxical approach, aka desensitization. Instead of trying NOT to imagine their having sex together, make it a CHORE for yourself to imagine their having sex together every time you eat. Or go to the bathroom. Or brush your teeth - tie it to something that happens several times a day. And if your imagination runs wild outside of those designated times, occupy yourself with something and tell your brain - look, we have a session for this coming up in 45 minutes when I will be having dinner. Right now, I am busy. Then, we will imagine all the things the ex friend/gf are doing.

You might become bored after awhile, but most importantly you will be facing your imagination rather than shunning it (which will not work either way).

If you can manage to graciously reconnect with both of them as friends, they will have enormous respect for you since you would come out victorious out of a challenging situation. And feeling respected is so much better than feeling vengeful!
Good luck!