LW: This pandemic feels neverending, no sight in end, no clarity. I'm OK most days but I worry about my adult kids and others who are struggling and sometimes I'm just so sad.

HAX (updated answer for today, not last summer):
Understandable. This is just hard, especially the light we thought we saw this summer was just more tunnel. But this is not a bad dream, and it is not never-ending. It's reality. Gotta work with it.
Don't want to be too dark, but people always will be struggling. Our obligation there is to be mindful and prevent further suffering. You can tend sick relatives, educate, vote, volunteer, compost. Whatever... as long as it's action that's meaningful to you.
But thinking about all the suffering all the time will drive you crazy. It sounds like your sense of duty and kindness has hit reality, damaging your peace of mind. If the sadness bears down regardless of what you do, please talk to a doctor in case you have clinical depression or anxiety.

Try to take the historical perspective on hard times being part of human experience, war, pandemics included--, and then ask yourself what you can reasonably do today. if it works, add it in, and make room by omitting what's not working for you.

Otherwise, seek out life-affirming actions to make you feel better: dancing, exercise, generosity, entertainment. Remember this pandemic is terrible because it is life-threatening. That fear signifies how precious life is, and how it is to be cherished/relished.

Me: LW, were you a habitual worrier BEFORE this pandemic? If so, I would suggest you do seek therapy for more appropriate ways to deal with stress. Keeping mentally busy and exercising is always good to help that free-floating anxiety.

More practically, as Hax says, if you learn to identify something you can usefully do that you enjoy and makes good use of your values and talents to help others, you won't have time to worry about what you can't control, not when you HAVE things you can do and control.
Stoicism is a useful philosophy for these who tend to worry, as well, done right.