I can`t get any further, cause the hacking mini game seems to be impossible. I always die in cyberspace.
What am I doing wrong?

i'll assume its the maze like section not AR mode

how are you dieing exactly?

if you start losing too much focus you can run back to your entry portal and leave,
and go back to tony's to use the recharge box thingy

Dunno what you're doing, but this thread contains some helpful tips:
https://steamcommunity.com/app/269650/discussions/0/490125737479090735/

It's also worth a try watching a Let's Play to see how others are doing (even if the guy I linked is having problems too ;-)

When buying disks, stick to just one or two types. It's better when one or two weapons are more powerful then all three being weak. I like to use Wedge for focused fire power and Pulsewave for clearing swarms.

Use both hands. Simultaneously aim & shoot with the mouse while moving using WSAD.

Always have some single-use software for emergencies. I like to use Datashield. It's cheap, and effective for escaping out of tight spots.

Keep recharging Focus. Leave cyberspace, recharge Focus, go back in. Everything you've destroyed will stay destroyed and is not going to appear again.

It's a bit hard in the sense that the game throws all those advanced cyber enemies at you very early into the game. Those red spawning enemies (virus nests?) are the most annoying. They spawn cannon fodder at such a rate that it creates a shield around them and I can barely get a hit in. If there are two of those near each other, they overwhelm you with smaller enemies in a matter of seconds!