Hi ! I activated the shrine in Fellglow Keep, the one that brings your health back. I used to have the blessing of the Lover's Stone.

After finishing the dungeon, I thought I wanted to have the Lover's Stone blessing again, I felt it was more useful.

I fast travelled to the shrine and tried to activate it, the notice said , "You already have this blessing".

Now I'm confused... Questions: Does activating one shrine "de-activate" the earlier one?

Once you activate a blessing, you can NEVER re-activate it again?

Thanks for your replies!

Shrines and Guardian Stones are separate critters, using the former doesn't invalidate anything you're getting from the latter. Using a shrine of one of the Divines gives you a temporary blessing associated with that Divine but doesn't interfere with whatever bump you're getting from a Guardian Stone. Guardian Stone blessings, once activated, are permanent until and unless you choose to activate another Stone. You can still get a temporary bump from a Shrine of one of the Divines and heal any diseases without losing the Stone's blessing.