I have studied Ethics in high school, but reading a whole book was very difficult. I studied Ethics in high school, but reading a whole book was very difficult.

"I have studied" is correct, but dropping the word "have" sounds slightly better.

I have studied Ethics in highschool, but reading a whole book was very difficult. I studied Ethics in highschool, but reading a whole book was very difficult.

Moreover, I don't have any bible knowledge, so it was hard forto understand the values of classical European values. Moreover, I don't have any bible knowledge, so it was hard to understand classical European values.

Moreover I don't have any bible knowledge, so it was hard forto understand the values of classical European. Moreover I don't have any bible knowledge, so it was hard to understand the values of classical European.

I maybe struggle to read the vol.2. I may struggle to read vol.2.

You can also say "I might struggle to read the second volume"

I maybe struggle to read the vol.2. I may struggle to read vol.2.