Hello MJ16.
First, you should know that your feelings are extremely common. Almost all men feel a profound sense of emasculation, humiliation, and shame when their WW has a PA. In most cases this impairs their desire and ability to have sex. The mind movies are a real erection-killer. It also sounds like your WW pined for him for a time.
Second, it is especially impactful where the A was long-term and was discovered by happenstance, like yours, as opposed to where she quit the A voluntarily and confessed. The A would likely still be ongoing had it not been discovered. You feel like you are her sexual Plan B. Her Plan A was her AP. She invested a great deal of time, energy, imagination, and desire into planning and implementing sex with him. Possibly way more than she has with you.
You should not make any decisions quickly. However, the first thing you should to is read here about The 180 and implement that in your life. You should stop having sex with your wife if you feel uncomfortable doing this, and you should even separate somewhat, such as by requiring her to sleep in the guest room or such.
I think you should also read "How To Help Your Spouse Heal from Your Affair". The book is written for a WS, but it is helpful for a betrayed as well.
Also, to to the Healing Library (top left of this page in the yellow box) and read stuff there.
Here is your new reality: the woman you thought you were married to does not exist. The main first task of a betrayed is to come to grips with this reality. It's like being hit on the head and kicked in the balls at the same time. Your wife is a woman who will fuck another man repeatedly for years and lie to your face about it. These are despicable, cowardly acts. One of the main questions you will want to answer for yourself as time passes is whether you wish to remain married to a woman who has proven to you that she will do these despicable things to you.
Another thing you need to come to grips with is the simple fact that she is a practiced and skilled liar. An express part of your conversation with her should acknowledge that, as you are coming to grips with how easily and profoundly she lied to you, for years, how can you ever know whether you can believe her?
Finally, you should let go of any desire for any particular outcome. D and R should both be options you weight, and make it clear to your WW that D is a route you are planning unless she proves to you that R is worth the effort. Both D and R are ruinously difficult.
For the time being, do not waste your time with MC. In the end, R only works if your WW figures out what is broken in her, fixes in, and convinces you that she has become somebody you would now choose to be married to, knowing that in her past she lied to you and cheated on you for years.
Among other things, she must be able to convince you that sexually you are not her "Plan B". This is an ephemeral matter of the heart that is not easy to describe with words. To this end, there is a wide range of discussion here on SI about getting the "dirty details" from her. I think most male betrayeds end up wanting every minute detail, under the thinking that if there is going to be healing, it's easier to heal from the known reality than from your imagined horror story. To that end, most recommend that you require her to make a detailed written timeline of the A, including how/where they met, when the A started in terms of attraction, when she decided to become physical with him and why, and when/where/how they met up each time, including sexual positions, etc. Also have her describe whether she bought/gave him any gifts, or vice versa. She should throw away any gifts she still has that he gave her.
Let me remind you that your WW is a known liar. She has been lying to you for a year, probably more. WW's always minimize their sexual experiences with their AP. ALWAYS. And they engage in what we call her "Trickle Truth". Do not accept any minimizing. I can't begin to tell you how many WW's have said things like "he wasn't really all that good in bed," or, "he had a hard time keeping an erection," or, "his penis was small," or, "he didn't spend any time trying to satisfy me, just in-and-out and he was done, he doesn't know my body and how to please me like you do." In the end, she invested a lot of time and energy and probably money into fucking him, repeatedly. Clearly she was enjoying the sex.
Good luck.
[This message edited by Butforthegrace at 1:21 PM, July 17th (Tuesday)]