MacBook will not boot beyond gray Apple logo screen

The MacBook in question is a 2011 13-inch MacBook Pro running High Sierra. It has all original parts still save for the RAM which I upgraded roughly 5-6 months ago. My MacBook was acting a little laggy and I will admit I was impatient after about 10 minutes of fiddling and waiting I held in the power button for a hard reset or restart (which ever is correct. Forgive my poor computer vocabulary) I went to boot it back up and it took it about a minute after it dinged for the Apple logo to appear. It proceeded as normal pulling up the prompt for me to put in my password and login like normal. I typed it in and the loading bar came up and it made it about a third of the way across when it rebooted itself. It dinged again and this time displayed a message saying it had encountered an error and to press a button to proceed and try again so I did. This began a loop of the exact same thing happening. I was finally able to boot it into recovery mode and ran a first aid check. It didn’t find anything wrong and so I tried again and same problem. Now though it won’t pull up the prompt for me to input my password. It boots, the gray logo comes up and it will load about half way before rebooting itself saying the same error message. When I go in to attempt to reinstall the OS I’ll select my hard drive and then it will disappear and reappear as grayed out and when I try to select it it say it cannot be used because of FileVault decryption. I’ve tried just about every guide online I can find and I understand that part of the problem is probably my MacBook’s age but I just really need some help solving this. I’m not in any position to give up and buy a new one as I’ve been told to do elsewhere.