MLC with young personal trainer

[BeingMe]

Okay, just my opinion, but it seems to me your W has been taking care of herself pretty well, recently. She is in the 'poor me' mentality, and it's hard to defend against, so I suggest you just validate her. You could say, "honey, I know how hard you have worked for our family, and how much you have sacrificed. I appreciate it, and I want you to be happy. So, it makes me happy that you are able to take some time each day to just concentrate on yourself". In her mind, she has done everything for her family, and nothing for herself, however, that doesn't mean she should just throw it all away on a whim, but you can't really tell her that, unfortunately. Anyway, here are some things you can try, if you feel you can (you choose what you think will fit your own beliefs, and personality):

Don't react, just validate
Work on yourself - GAL, perhaps get a personal trainer, and work out at the gym too. You also need time for yourself, and an outlet for your frustrations. It'll help you to sleep at night too. Work on being the best you that you can be, but don't beat yourself if you come short sometimes. You are human, just as your W is.
Don't make any assumptions, or have too many expectations. For one, just because she has feelings for this 24 year old personal trainer, doesn't mean he reciprocates at all. I am studying to be a fitness trainer, and we are there to listen to our clients, and focus on their training needs, and your W may've misinterpreted their R, making it more than it really is (much like a patient may with a psychologist).
Don't let the children hear any of the things your W is saying about having enough of them, etc. Kids can internalize that, and make it all their fault, and it isn't. She chose to marry you, and start a family, so she is their mother, despite what she is presently going through.
Do not tell her she is in MLC - to them, this is real.
Do not tell her you love her, unless she says it first. Don't start R discussions, but you can respond when/if she does.
Do not pursue, beg, cry, or whine. That is very unattractive, and won't help you get what you want.
Try and be upbeat, and positive. Look for things to laugh at ... movies, comedy channel, little things the kids do that are cute, etc. Laughing is a great stress reliever.
Try some 180's, such as responding when she is masturbating, and see what happens. Otherwise, try little 180's, and if you get a positive response, continue, and try something else.
Finally, pray, pray, pray ... for patience, for clarity of thought, for acceptance, and the ability to rest, and leave it in God's hands. Do what you can, but in the end, none of us have control over the actions of others, only our reactions.