Since High Sierra is the end of the line for that Mac – and won't run current commercial applications from some vendors such as Microsoft and Adobe – you might want to consider:
(1) Getting a newer Mac, or
(2) Getting applications that are available for High Sierra, while they are still available. For instance, you may be running into the problem that you can't upgrade Safari, and some Web sites no longer support High SIerra's old version of Safari. Firefox is still getting updates for High Sierra, and using Firefox for some Web browsing might help with that issue. You can't get Microsoft Office or 365 for High Sierra, but you can get LibreOffice (for now). Photoshop no longer runs on High Sierra, but GIMP will.