MacBook Pro not getting past log in screen.

I got my MacBook Pro when it was new back in 2015. Here recently I noticed it wasn't allowing me to save any documents, so I restarted it thinking that would help solve the problem. Well I was wrong. Ever since then I haven't been able to successfully log in.

After I enter my password, it gets through about halfway loading, then on the left side of the screen white lettering on a black background pops up, resulting in it to restart completely. No matter what I do I can't get past this process, letting me know it's the hard drive that's having problems. I have tried talking to the online apple staff (including a supervisor) for help and after everything they tried, nothing worked and it's resulted in me having to take it to an apple store.

I am just seeing if anyone would happen to know any way I could use my older iMac desktop to somehow retrieve everything from the laptop without going to an apple store (since the closest one to me is an hour away). I just mainly don't want to lose everything on the Pro because I am a noob, and didn't back it up with Time Machine or an external hard drive.

Thanks in advance to anyone who tries to help me!