One way I stay engaged is to only buy or unlock stuff I intend to use. I am only missing a couple of engineers, but unlocking them is only a completionist thing for me right now. One day, they'll know my presence.
Play your way, but having an end goal similar to other games--like I have for Destiny or similar titles--is something I moved away from. I decided to just play and think of what activity sounded fun to do, without too much focused grind. I already got the big 2 navy grinds done in order to enable myself to truly do this.
I secretly want to take a vessel out across the galaxy and explore one day. I'm just waiting for the Beyond series of improvements to do so. Instead, I'm getting combat practice with different weapons for now. I'm also planning to engineer an Imperial Eagle for speed and only speed to see how much fun I can have incinerating cliff sides. XD
TL; DR: Think of what drew you in to elite in the first place. Think of what sounds fun to do, and do it. If you're like me and need goals to accomplish lots of the time, it'll be easy to overlook those things, but try. Or take a break.