MacOS Login screen repeats endlessly

I had a serious problem with my system crashing every 6 to 8 hours - a look in the console showed warnings about memory leaks - and one process 'quicklook' reaching an "Endpoint" literally thousands of times before the system ran out of resources and crashed. I was forced to try repairs, and then ultimately reinstalling my OS ("Sierra") over the internet, using Mac Recovery.

Now I get a login screen with my username (and a symbol I wasn't using before as my 'picture') - If I enter a wrong password deliberately: it rejects it. When I enter the correct password the screen starts to change resolutions and the cursor size changes ... then the screen fades to black and the exact same login screen appears again.

Please login ... Thank you / fades to black (Repeat endlessly)

Before I used Mac Recovery I COULD Use it - knowing it would restart every 12 hours at most when it crashed. Now that problem MAY BE gone - I can't tell because I can't login. Starting up in Safe Mode didn't help - just no background image appeared while I was on the endless login treadmill.

I feel like the recommended procedures made things much worse for me because I had to ask a question to the community from the browser in the recovery partions "GET HELP" feature - and can't USE my computer any more.

I used Mac Recovery and now I have a doorstop that endlessly asks for the apparently correct password - what do I do now? Thanks for any help you can give.