@OP one thing I would say is that in my experience having your head turned by someone new is always more appealing – the first flushes of love, the physicality, the freshness of a new relationship means the new person can do no wrong in your eyes. You don’t have to deal with the drudgery of life like putting the bins out or cleaning the bathroom, or discussing what’s for tea, sorting out the life insurance etc with this new person, and if they feel the same way about you then it’s a massive ego massage too. Thing is, the new person soon becomes a fixture in your life and the same mediocrity will return 🙂
I guess I’m saying don’t take what you have got for granted, but ultimately if you’re miserable and can’t see a way forward, maybe calling it a day is the only option?