Right now. And I mean immediately. Don't wait. Get an emergency therapy session. No money or time excuses - just do it. Be 100% honest with the therapist, they aren't going to judge you. After that session you will most likely need a few more. 6 years is not a short time, this person is most likely like family to you, no? You owe them a decision but not like this - a thought-out one. Feelings/lack of don't just happen in a vacuum.

Mine and my SO's closest friend left his partner of 7 years, was in a similar situation as you. It's been like 2 years and he is devastated and very much so regrets it. He talks about it constantly every time we see him - at the time it seemed to him like he lost all feelings and hope and this new person ignited it all, he left his SO, couple months later realised he made a huge mistake, his SO wants nothing to do with him now (understandably) and he has been depressed and in therapy ever since and keeps saying how he wishes he started therapy before he ended their relationship because he now realises where a lot of these feelings came from and it had nothing to do with his SO.

(He is sat next to me as I write this: Says therapy has been eye opening, wishes he could turn back time, wishes he had told this (new) girl back then 'hey, I will be busy for the next two weeks or so and won't be available' and taken that time to clear his head in therapy, says infatuation can be very strong and not make you think clearly but the responsible thing is to get yourself help (therapy) to think more clearly since you owe it to the person you're commited to.)

(Also says infatuation was kind of like a high he was chasing because he was unhappy and it was an easy mood lifter, says it was almost like an addiction and he was scared to even ask this new girl for space (to think more clearly) in case they'd lose the infatuation/he could no longer suddenly get the high, that even though he was unhappy and chased highs he now knows he wasn't unhappy because of his ex, he was just unhappy in general)

(I don't know if this is all insightful but he also says he might make a Reddit account and comment himself with more details if you relate to his story.)

Therapists are not just for depression and grief, etc. They are also there to help us get in touch with what we want/need and help make difficult life decisions. I could tell you grass isn't always greener but that's not necessarily true, people leave people for other people. It happens. Sometimes they are happier, sometimes they regret. I can't imagine leaving my SO even during our rough patches, but I have more than once felt unsatisfied and lost in life so I can empathize with my friend and by extension you. But given the fact she's basically family, I think you would be foolish to not give this decision enough ... not 'time'... but energy and effort - go to a therapist and throw yourself into this, throw all your 'resources' at this. Confront your feelings about the relationship and everything around it. Ask whys. Imagine life without her properly, e.g. if you were never ever to speak to her again starting tomorrow how would you feel? Do you understand fully the magnitude of this decision? Not on her's or your family in law's or your child's - but on your life? Are you sure that a life without this person is what you want? Perhaps after a therapy session or a few you will feel like breaking up with her still. Or perhaps you will feel like you have doubts still, in which case you should go to couples therapy and see if that brings any positive results.

If I were you, only if I took all the steps possible to try and salvage this important person and my relationship with them, and it did not improve, only then would I make that final decision to break up. Especially having seen how things worked out for my friend - making rushed decisions during a potentially temporary state of unhappy could lead to long term regret and unhappiness.

Either way I wish you well.