To think it’s clear as day now that Brexit was a massive mistake and we need to rejoin the EU asap

But that was the whole point, from Day 1!!!

"We must build a kind of United States of Europe", Winston Churchill, 1946

The statement "Have you seen how badly the economies of all the countries in the EU are doing at the moment?" is not factually accurate as written.
The key issue is the phrase "all the countries in the EU." EU economies are not all performing badly, and their performance varies considerably.

• Recent data show that the euro area economy has continued to grow, albeit modestly and with significant headwinds such as higher energy prices and geopolitical uncertainty. (Reuters)
• Some large economies, including Spain, have been growing relatively strongly, while others, such as Germany, have faced weaker growth. France and Italy have seen modest expansion rather than outright collapse. (Reuters)
• Major institutions such as the IMF and the European Commission have downgraded growth forecasts, but they still project continued (though slower) economic growth for the EU overall, not a broad-based economic crisis across every member state. (IMF)

• "Have you seen how weak economic growth has been in parts of the EU recently?"
• "Have you seen how some EU countries have been struggling economically at the moment?"
• "Have you seen how the EU economy has been slowing amid recent challenges?"