Is it just me or is this game hard, even on normal difficulty? Especially the optional side-missions.

Trepingu!
You might want to elaborate and provide context on what you're having trouble with.

The endlessly spawning enemies in the hacking side mission.

The game has a bit of a learning curve, but I didn't find it too hard on lower difficulties short of a couple sequences. Most of the hard parts are wide open spaces where there is no cover because a lot of the guns in the game are not particularly accurate.

Only general tip I can really give is to make use of all the resources the game gives. That means both bullet time AND cloaking. I only used bullet time to start myself.