In my personal opinion, phrasal verbs areis one of the most difficult subjects to learn.

“Subjects” must be plural if you are talking about “one” subject that falls into that category.
I think it makes more sense here to treat “phrasal verbs” as singular because you are referring to phrasal verbs as a topic, rather than as a set of verbs.

In addition, irregular verbs conjugation is also a challenge.

When using a noun as something like an adjective, as you do here with “irregular verb,” the noun should be in the singular.

In both cases, I must learn by heart,; there are no tricks or shortcuts.

Beware of comma splices (an error where you separate sentences that can stand alone with a comma, rather than with an appropriate choice of conjunction or punctuation).

Some websites offer services to help onwith that, like online quizzes or exercises.

Online tools help to reinforce, but it is necessary to memorize before using them.

We need to know what “before” refers to, and with the original phrasing it is unclear.

I also make mistakes regarding to the use of prepositions.

“Regarding” is not followed by any prepositions, especially if they are not part of the thing that is concerned.

I borrow grammar exercise books with answer keys from the library, to improve on that.

All the previous points were related to writing in English language.

Or: “in the English language.”
“Points” is grammatical but feels vague and imprecise when used in writing. “Examples” or “situations” would be better.

When it comes to listening, the fact of havingexistence of different accents can make things worse.

I try to train my ear to understand people from one single region or country (pfor example the UK).