If you’re seeing a long-term decline in your traffic from Google search, the first thing to check is if this is caused by one or more high-traffic pages falling down the rankings. You might get a clue here from a snapshot in Google Analytics of the top landing pages from Google search on same week in previous years. Otherwise, is the fall in traffic steady, and across the board? If your site hasn’t changed much, it probably boils down to one or both of the following: your competitors doing better, or falling the wrong side of Google algorithm updates.
Either way, it’s time to up your game. A quick site audit might be a good start. I like to look at some high-profile search terms (I look in Search Console for those with decent volume where my site averages a position between 5 and 20). Now investigate the top results, and see if they have attributes which your page doesn’t. I like to compare things like page titles, headlines (stuff in H1 tags), word counts, description meta tags, links from other sites, and links from elsewhere in the site.
Observing the competition can provide you with some great ideas which can be applied to improving your performance on a particular search, or across the board.