year 3 was when the cracks showed for us... anyone else hit a wall around then

first two years were honeymoon phase 2.0. amazing. year 3 something shifted. we started getting complacent, not checking in, taking each other for granted at parties. had a really bad night in glasgow where i flirted with someone she'd told me earlier she found intimidating. didnt sleep with them or anything but the breach of an unspoken agreement nearly broke us. took therapy and a 4 month pause to rebuild. now in year 5 and stronger but year 3 was rough. is this common or did we just get sloppy. wondering if theres a typical 'wall' point in the LS marriage journey