How can I go back to Watch OS 9?

You cannot.

Express your discomfort at https://apple.com/feedback

I believe it is possible to have an Apple Store send your Apple Watch back to Apple to have it downgraded (or at least for short period of time).

But that is just a short term solution, as you will want security fixes and that is not going to happen on older operating system versions. Plus eventually the iPhone will no longer support older watchOS versions, and you will either have to decide to not upgrade your iPhone, and have more security fixes (most likely more serious to your personal data) not be applied, or forget about the Apple Watch dependency and upgrade anyway, only to find the Apple Watch does not talk to the iPhone any more.

I agree 100% with you. Now the side button is absolutely worthless to me. I never, ever, go into the Control Center on my watch. I much preferred the 9 version where it would bring up your favorite apps, which I use multiple times a day. And the action button is still so limited that you can't do much with that. Why can they just not let us choose what we want these buttons to do? Why not have a switch to allow it to operate like it used to in watchOS 9? Too difficult to allow people to have what they want and customize it? Must force them to use what you want them to? Time to sell the apple products and go back to Android.

I understood you could have Apple remove OS10 beta for a free, or they can do it after a service if you have an older iphone, but I doubt they would do it out of some preference for the old days

a tad impetuous, but feel free.