I’ve had the same MacBook Pro (late-2011 model 8,1) since early 2012, and am no longer able to put new versions of macOS on it, as Mojave only works with newer models. Over the years it’s become much slower, and the 500 GB HDD that came with it – almost being full – is near failure, according to EtreCheck Pro.
For a reasonable price I ordered a 1 TB solid-state drive and data transfer kit from OWC and within a few days I was up and running on the new drive.
Between following the suggestions on EtreCheck Pro (like deleting unused 32-bit apps and uninstalling unnecessary daemons) and this new larger and faster drive, it’s like I have a brand new Mac – except that I still can’t install Mojave. Unfortunately, there’s no fix for that.