I absolutely agree with the OP.

Honestly, I did try to get used to crowns but it was so hard and confusing for me to have to deal with dozens of new words at a time in my Irish tree (even with the help of the Memrise course) that I have finally given up and installed an outated pre-crowns version of the Duolingo app. Now I am back to the strength bars and can practise each individual chunk of a skill as long as I need to.

It feels great not being forced to tackle new words and structures before I've had a chance to actually learn the previous ones!

I have been learning Russian for over two years now on Duolingo but due to the disappearance of the ability to do an exercise with a pre-defined set of words etc. I feel my knowledge of the Russian language is actually decreasing. When I press the "practice" button 7 out of 10 times I keep getting words etc. I already know, but the 3 I don't know don't pop-up often enough during the exercise to learn them out of my head.

This was easier to learn when the ability to learn a set of words was still available. I really hope this feature returns at one point as (for my feeling) being able to repeat such exercises is essential in learning a language.

I too have become discouraged with this new format after attempting to learn German for some time and achieving a 68% fluency score, your format changed and I was unable to repeat previous lessons. Being the type who learns through repetition I was very discouraged and quit. I am now trying to learn Spanish but am again disappointed . please help

Yes, lots of people have already complained about this missing feature in the new "crowns method"
Unfortunately, I don't know a workaround at the moment. You might have a look at the lots of discussions about the "Crowns" and upvote the ones that you like.

It might be very useful to read this discussion and to write a comment about your experience and wishes.
Our Crown Levels blog post is live! https://forum.duolingo.com/comment/27967582