So, reading all 5pgs seems quite painful (especially when everyone is saying cheating, leave etc etc), but here's my contribution from the other side of the fence:
When I first started dating my SO we were on fire in the bedroom. Thing were awesome and I was head over heels over head in love with him, his body and our life. We moved in together and life was awesome. Then things got intense for me at work and I would come home absolutely wiped at the end of every day. Being so on edge made me mentally nitpick every single little fault of my SO. as a result sex and intimacy dropped to zero. Eventually it got to a point where weekends were spent with him in front of the Xbox in the lounge and me watching movies in the bedroom (European winters are not built for many outings outside the warmth of your flat). Things eventually culminated to a point where he felt so isolated that he cheated on me and I left him. The sad thing was that our lack of intimacy wasn't because of infidelity at the start but because of a lack of effective communication.
She might not be cheating, but that doesn't mean she isn't unhappy about/with something. DONT cheat on her. It's a heartbreaking thing to do. Talk to her, give it a chance, throw in a few gallant gestures (something out of character and classically cheesy romantic - girly surprises are always a win) and hope that she sees and feels your efforts.
If she still loves you, she'll warm up again.
Things can get better from here. They have for me and my ex ex (we got back together

), and I don't see why it can't for you guys.
Good luck!