MacBook Pro system utilities restart loop

I let my computer install new updates for my 2011 version MacBook Pro unaware that it also installed High Sierra from the Apple Store (installing goes manually, however it does not show what it installs). After two days of testing I'm unhappy with the addiotional installing of High Sierra and I decided to downgrade my Mac back to OS.

I was able to recover simply by deleting High Sierra trough system utilities, however now my Mac is stuck in a restart loop! It shuts down normally and restarts creating grey bar loop notifying that it goes back to the system utilities menu. I have restarted it normally more than three times and not sure what I should try to next.

This is giving me high alert red warning lights since similar behavior happened back when Sierra update which destroyed my computer. Let's say that I'm not happy to pay another two thousand euros for another repair run from another failed product. So I'm hoping to get this fixed without having to go for yet another shop with horrible customer service. I have already searched and digged stones around the forum but was unable to find anything with similar sort of problem.

So how should I proceed in this kind of situtation?