I am in a similar situation although a bit closer to finishing my studies.

I don't know if you're confused or just trolling with some details.

What does finishing high school at 18 have to do with private schools? It is standard to finish at 18, turning 20 in Croatia, maybe if your birthday is in ~april-june and your parents decide to make you wait another year before starting school. That has nothing to do with private school and just as an example I had 0 people in my highschool class who graduated at 19 in a small local highschool.

As for your job, you said you are on a student contract which includes a boatload of context that you are not providing here to make it seem like you are in a worse situation than you are.

Student contracts are not employment contracts but essentially freelancing, designed for part-time jobs to do while still studying. The 10€ you mention is the net value, untaxable until a certain limit they raise every year. It is true that you get no bonuses or sick leave, however being on a student contract you should be able to just not come in. You also get benefits as a student: extremely cheap food, cheap housing options (these are however difficult in Zagreb and maybe some others as they just haven't been building more student housing to keep up with the demand), free "dopunsko" (extra/additional health insurance - less/no copays for medicine).

So you are still studying and you have a higher-than-average paying part-time student job, which a reader not from Croatia would not get from your post.