OP - It must be painful to know the OM has ultimately chosen his wife over you after so much time and love between the two of you. No one "has" to stay married. He could leave and he hasn't. Six years. Do you feel used at all? Or is it all just so lonely because you have this great love that can't be discussed?
Leaving both men may be the answer. Have some alone time. Reflect on what happened. More married men will come along in the future (they are always around, available, willing and persistent) and before it starts, but after interest is expressed, you will stop and think, "Do I want to be this man's mistress?" You will be older and wiser and will be able to weigh that decision with maturity despite interference from lust or infatuation. Married men are easy. Sadly, it's up to the OW, or potential OW, to protect these straying married men from themselves.
But, that is you in the future. Now, you are in a bad place. You need to grieve the loss of this relationship without any empathy or support. So many women HATE a home wrecker. You get no sympathy from them. I mean, that is just the reality - the nature of the beast, really, not so much judgment call. (Perhaps this is not unlike what closeted gay people experience - they are unable to get emotional support for the demise of relationship no one knew they were having.)
Anyway, this is my first post on mumsnet. Long time lurker. Such great advice here.