Privacy errors / Security updates on Mountain Lion

Hi all,

I have a Late-2010 Macbook Air running OS X 10.8.5 (Mountain Lion). I am having a lot of issues with Chrome blocking access to websites because "certificates were signed by a untrusted issuer".

After a lot of research, I finally managed to solve one of these issues, by deleting and reinstalling DigiCert certificates. It opened access to some websites, but some others are still blocked, apparently because of another certificate (Symantec).

Now I'm afraid at this space I am going to spend my life trying to repair certificates. Is there something else I should be doing? Is there a way to perform a Security Update for Mountain Lion in 2017?

I am not really interested in updating to Sierra since the computer is old and it would be too much of a load for it.

Any ideas or pointers would be much appreciated. Thank you!

-Mat