Looks pretty obvious from the processes; you're consuming too much RAM, for your server specification.
Remove varnish and run mysqltuner - follow its' advice (apart from cache nonsense).
Fine tuning of nginx/Apache also required. With a reverse-proxy nginx, Apache can be paired back significantly if not serving too many requests to mySQL/maria.
Think that you also need to reduce keep-alive timeout, if indeed you enabled it, which you should.
All standard stuff required of a system administrator.