I am only about 1 month post bomb drop, so let me give you a disclaimer that it is entirely possible I don't know what the heck I am talking about, but maybe I do. Here are my thoughts:
For me I think what allowed me to let go of a lot of my fear (certainly not all of it, but MOST of it) was sticking to the FACTS.
Fact #1: Lots of people in my life love me and admire me just the way I am. In DB'ing we are actively working to be EVEN better. If H determines for whatever crazy reason that he would rather NOT be with someone as awesome as me then it will truly be HIS loss. The same goes for you. I am certain you have people who genuinely think you are terrific and lovable just the way you are. Work to see yourself through their eyes and know that no matter what happens you deserve love and happiness and you are completely capable of having it with or without the H.
Fact #2: Probably unique to my situation, but perhaps helpful to others too. My H fell into his infatuation with a woman who appears to have a ton of her own issues and already he has made comments that makes it seem like he is realizing she isn't perfect after all. It probably helps that he "fell in love with his soulmate" knowing almost nothing about her except what she looks like and that they have a hobby in common. Now that he gets to face all the "glamor" of hanging out in her trailer house that doesn't really get cleaned much I think he is beginning to appreciate that it DOES take work to keep a household together (I am no great housekeeper myself, but I guess I am better at it than her and I have made it one of my 180s to be more tidy and organized) and she is probably not a prime candidate to fulfill all the responsibilities in life he is used to me filling. Especially since she is just like him, living it up like a child and shirking almost all responsibility at the moment. :P
Fact #3: The number of relationships that start as affairs and actually end up in "long term marriages" (over 5 years) is something ridiculously small... like 2.7%. If there is an OW, the odds are not in her favor in the long run. If you can play the waiting game you have a good shot at getting the H back, if that is what you want and you can handle the aftermath of OW with strength, confidence, and grace. Also, 60% of second marriages end in divorce... so don't expect that H really has found his golden ticket. Especially in the context of MLC, they are usually running into the arms of someone else blindly and as time wears on the illusion starts to lose it's appeal. There is the chance they may fall into serial affairs, but I think DB'ing puts you in a strong position to be there when they fall out of that first one and get them to realize you really have been there loving them unconditionally all along and the grass will never be greener than that.
me-35 WAS-37 T-16 1/2 Son-14 (HF Aspergers) BD,ILYBINILWY,"I met my soulmate": Oct5,2013 "Letting go because I love him, holding on because I love him."