Thanks for sharing some of your story, and for understanding. I also have depression to play with and possible C-PTSD, so that complicates things. I can only imagine how bipolar makes things harder for you, too. Yes, there are a lot of mental illnesses that co-occur, so we often get hit with a lot at once.
I wondered for years on and off if I had some sort of early onset dementia, my working/short term memory have been so, so poor. Thing is, no-one takes that sort of thing seriously. If you mention instances of forgetfulness, it's always a big laugh or "Oh, that happens to me, too". I mean, sure it does, but half a dozen times before breakfast? Every day? Finding out what was going on and getting a diagnosis were something of a headache.
I try doing one task at a time, but that may or may not work. There's an endless loop of spotting other jobs around the house that need doing and thinking "Right, better do that before I forget" - and as far as that sort of thing goes, my impulse control is rubbish, so off I go on the new task. Completely relate to your story about missing that gathering. Recently for me it was trying to remember to go and say goodbye to one of our managers at the end of my shift, because he was being transferred to another store. I had my reminders, and it was in the back of my mind for the whole shift - though so was a list of groceries to buy after work - so in the end I remembered the groceries, forgot to farewell said manager. D'oh!
I do find where my self esteem suffers is less in the annoying little incidents than the overarching malaise. I average about 20 hours a week work, and manage a household with my partner, who helps. Staying on track and functional at work and then at home, I do actually get things done, but the sheer amount of effort, willpower and energy it takes leaves me exhausted as if I did full time work and had no help at home. So of course others work full time, have partners and kids, do social things, and they seem fine. To me even half of that is overwhelming and anything fun after the bare necessities often ends up being more than I can manage. The self care goes downhill from sheer lack of energy, and my sense of self worth with it. I feel bad about myself because I cope so poorly with basic life stuff. Can't get away with living on canned spag and not cleaning like I did in my 20s, got two little birds and an immune compromised man to worry about. Aaargh!