First up, i think it’s normal to have these highs and lows of riding enthusiasm; currently in my 4th(?) ‘bike high time’, I try to recognise them for what they are and don’t sell all the bikes.
For me, fitness and bike fit are important for my enthusiasm. If i’m not feeling fit and every ride is a struggle, i’ll start to think about other things. When my bike was a poor choice that never fit right in the first place, that has had an impact on me too – you mentioned back pain? Anything you could do easily, like a change of stem/ bars, to make it more tolerable?