What MJ wrote. "What next?" should be your mantra. Anytime you sit there staring at the instruments you need to be thinking about the next few seconds, the next few minutes, the next few tens of minutes and the end of the flight - in roughly that order of priority.
If you're siting twiddling your thumbs then you've missed something.
Become *very* familiar with the avionics. The more efficiently you can get to whatever page/data entry you need, the better.
Similarly with the 'rhythm' of a flight. After take off 'these actions' need to be done, TopC 'these tasks', in cruise the things to prepare for the next leg/descent/approach, before TopD = ATIS &approach brief, prior to FAF for pre landing or pre approach checks etc. Those things are largely the same each flight and should happen as a matter of course.
During an approach I have two parallel thought processes: The next few seconds or leg, and where I am in the overall picture. For the next few seconds or minute I use 'T.A.A.' as a mnemonic: next (Track, Altitude, Aid)
* Track: current & next track or hdg + time/dist limit
* Altitude: Next target, next limit
* Aid: Flags away or NDB ident still present, approach mode/RAIM/WAAS for GPS
I do this as soon as I'm established on a segment. If you like, I focus on the current track/altitude only enough to maintain & keep under control but am really thinking about what happens next. In my mind I keep saying the mantra, over & over...
Example.
Teardrop entry: inbound HDG aaa, next TRK bbb 1 min. Cleared 4500' MDA 4000', flags away.
(now on teardrop outbound): TRK bbb 1:00 (minor thought), next LEFT turn intercept TRK ccc. Cleared not below 3000', flags away
Now inbound after teardrop): TRK ccc (minor thought), next TRK ddd outbound, 1min30. 3000' until xxx DME then 2000', flags away
Procedure outbound: TRK ddd 1:30 (minor thought), next TRK eee inbound. Limit 1500' until est. inbound then MDA 600'. Flags away.
Inbound: TRK eee 'til aid, next TRK fff. MDA 600', missed CLB 3000', flags away.
...and so on.
Trim! It drastically reduces your workload. If you can't control pitch with *light* fingertip pressure then retrim. Not squeezing the column with thumb & 1st finger together but a 'push' or 'pull' with one or the other. For large pressure inputs then heel of hand or 2/3/4 fingers for as briefly as possible until trim can relieve what pressure you need. Remember: Don't squeeze! Push or pull, but not squeeze.
Eventually you'll have a series of well rehearsed task subsets that are somewhat automatic: At this point do these things, that point those things. When that happens you'll have a lot more time to relax & enjoy the flight. Until then you have to consciously orchestrate what you think about & do which leaves little to no time to much else.
Oh, and don't forget the aircraft's automatics either! Learn to use them to support your workload management. But don't use them as a crutch - you still have to be able to do the flight without them.