MacBook Pro freezing, kernel panic, not booting up

Hey.

Yesterday, my Macbook Pro completely froze whilst I was using it (never done this before). I had to do a forced shut down. When turning the macbook back on I would get this flash of code appear over the white screen with the Apple logo on, it would only stay a few seconds, then my login page would appear. I would try to login, but after my first attempt, the macbook would shut down and repeat the same process (with the code appearing on the screen). I would then normally be able to enter my login details on the 2nd attempt, which would leave me with the loading screen with the progress bar - which would take ages and eventually freeze when the bar was showing to be fully loaded.

This was how the problem started. From here I tried various methods, from resetting the PRAM, SMC and booting up in recovery mode. When in recovery mode I eventually figured out how to perform first aid on my SSD (as it was encrypted, I needed to mount it as before it was greyed out). The attempt to repair it with first aid was unsuccessful. I ran it twice, and both times it gave different reports of what went wrong - not that I understand what the output info meant. I also ran a quick AHT - which produced an error - 4HDD/11/40000004: SATA(0,0).

During my numerous attempts of restarting my macbook, one occasion the flashing folder with a question mark appeared when attempting to start up. Now, when I try and start it, I just get the white screen with the Apple logo with a progress bar that doesn't seem to load, then my macbook screen just goes blue/black - then resulting in the need for a forced shut down.

I did eventually manage to boot up from an external HDD with a CCC backup on - although this was painfully slow. But I believe that might be due to having only 19GB free on a 1.5TB drive.

Macbook Pro spec: Early 2011, 13", upgraded 16GB 1333Hz Crucial RAM, upgraded 1TB Samsung EVO SSD. These upgrades were made probably 2 years ago. Never had a problems.

Thank you in advance for any help or advice anyone can offer. I've been searching all night for solutions and I am looking for what you guys think, and then perhaps a way of narrowing down the possible causes.