I previously owned that same machine (mine had a 252 gig SSD), and never had any issues. It was solid as a rock. I replaced it with a mid 2017 model (again with a 252 gig SSD), and just like the old one, it performs well.
To add some more questions to what Cory asked:
1. Have you ever done any disk cleanup/maintenance/repairs? You can actually do a good amount of disk cleanup on your own, and there are some excellent products available (both free and commercial) that can help you with such tasks.
2. This is
critical: are you making backups to an external device? If (hopefully) you are, what software are you using for that?
3. For the Mojave upgrade, are you just trying to upgrade "in place", or via a clean, fresh, "virgin" installation of Mojave? Given that you are having issues with High Sierra, a clean installation would be better.
4. No matter how you upgrade to Mojave, it is critical that you insure that any third party (ie, non-Apple) software you are using is compatible with OS 10.14.2. This link can help with that:
https://roaringapps.com/apps
It would be wise to get rid of Advast Anti Virus. Why do you have it installed anyway? There are some other, excellent products available that you can occasionally run to check for adware/malware/viruses.
To get rid of Advast, download the excellent freeware program AppCleaner, available from here:
https://freemacsoft.net/appcleaner/
Make sure to get the correct version. It does an excellent job of finding most files, folders, etc. associated with Advast (and of course the Advast software itself).
Two excellent programs you can use to look for adware/malware/viruses are:
1. Malwarebytes -
https://www.malwarebytes.com/mac/
2. ClamXAV -
https://www.clamxav.com/
Malwarebytes is very good at detecting adware and/or malware. ClamXAV goes "deeper" than Malwarebytes. I actually purchased ClamXAV, but it is not running in the background. I do run it occasionally (about once a month), and thus it does not bog down my system.
