Don't take it all out on one event. Sure, it's tough, but it's also tough to accept help. If it's not appriciated, well In any case you did your best and tried. That's enough.

An organization with some structure might be a better fit.

Community theater? They always need stagehands, set builders, etc.

Same for community bands and orchestras. A librarian or personnel manager for a group like that is as good as gold.

Conservancy? Help protect your local wild lands?

Find a cause you care about and an organization that works for its benefit and call them to offer some of your time.

I'm sorry that happened! Possibly he wanted to retain his independence as long as possible or sensed a threat (muggers offer "help" before mugging). It's always best to ask if you can be of assistance, and a simple, "Are you sure? It's no trouble," if declined is enough reassurance without being pushy. If the offer is declined a second time, a cheerful and sincere "Okay, have a great day/evening!" is an unakward way to take your leave.

I'm not sure how to volunteer online, unless you want to record audio books, you could upload to YT, or place a few free ads around to social media/local radio offering typing, proofreading, tech support, etc. But I have a feeling free tech support may consume more time than an hour.

It's good of you to want to offer your services. Thank you for that.

What did you learn from it about yourself?

Oh my. That's all that you can think of?