Everyone has periods of feeling down, dissatisfied with life (is this it?) with no energy to improve things. I think age has something to do with - you reach a milestone age and realise life hasn't panned out as you expected, you feel disappointed.
In the meantime everyone seems to be getting on in life, even enjoying it but of course we only ever see the mask that others want us to see. I'm sure you put on a smile and tell everyone you are fine, maybe embellish things a little bit now and again.
If these feelings drag on, you get so low that you start to neglect yourself, your friends and family and no longer have the energy and enthusiasm to do ordinary day to day things like keeping yourself clean, eating, going to work, it is time to see the GP.
If you feel suicidal at any time, call 111 or the Samaritans if you have nobody to talk to.
I hope you start feeling better in yourself soon, Abbey.