I've read this thread with interest and I've been thinking about all the issues raise, as I've been cleaning the house today. I do think that as you get older, it is natural to look back on your life and the decisions and choices that you made. However, if you're like me, you tend to focus on the things you got wrong, and ignore the good stuff. I was lunching out with a friend the other day, when I noticed that I had taught one of my fellow diners, now a middle aged woman. She recognised me and came rushing across, throwing her arms and around me and declaring : 'Oh, we all loved you so much!' I thought to myself rather sardonically that I wish I had known that at the time, but my point is that we do tend to put ourselves down and it is important to try to strike a balance. Ruminating on the past is not a good thing, I think, because we can do nothing to alter things and we probably weren't as dreadful as we thought we were.
As regards feeling grumpy or weepy or sad - no one can be happy all the time, so I don't think it's helpful to regard these emotions as anomalous. I'm not talking about true depression, of course, which needs medical help, but why should we expect to feel happy or chirpy all the time? Certainly, at my time of life, I have lost parents, partners, good friends, not to mention lovely pets, and there are days when I still grieve for them. I don't equate happiness with success, because bad days come to all of us, but emotions are temporary and the bad and sad feelings do go away. So I would say - embrace your inner grump, be kind to yourself and remember that tomorrow is another day, without any mistakes in it as yet, as Pollyanna used to say. 
to all who are feeling a bit down.
and 
for all those embracing their inner grump today.