My mistake <em>(We might say "my regret" which means I wish I had done something differently, but I am learning from it)</em>
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I'm tired of studying some difficult subjects for me such as informatics, physics, and chemistry. I'm just a girl <em>(unless you really feel this way as a girl, you might mean "I'm just a young person" or "I'm just a teenager")</em> and I'm not intelligent. I can't keep up with my classmate. <em>(see my comment to you about your intelligence, but you might express it as an emotion like "I don't feel as intelligent as my classmates") </em>My biggest mistake (<em>regret)</em> is specializing in INFORMATICS, specially <em>especially because</em> I have to study FREE PASCAL. If I had a wish which become true, I wish that I could return time to change my decision. <em>(In USA we might say "I wish I could turn back time" or "I wish I could turn back the clock") </em>At that time, <em>Then </em>I would specialize in Literature.
My mistake
I'm tired of studying some difficult subjects for me such as: informatics, physics, and chemistry. I'm just a girl and I'm not intelligent. I can't keep up with my classmates.
My biggest mistake is (or: was) specializing in INFORMATICS,
specially I have to study FREE PASCAL.
especially since I have to study FREE PASCAL.
or: in which I have to study FREE PASCAL.
If I had a wish which become true,
If I had a wish which would become true,
or: If I had a wish which became true,
or: If could have a wish that would come true, (<-- I think this one is best)
I would wish that I could return go back in time to change my decision. At that time,Then I would specialize in Literature.
My mistake
I'm tired of studying some difficult subjects for me such as: informatics, physics, and chemistry. I'm just a girl and I'm not intelligent. I can't keep up with my classmates. My biggest mistake is specializing in INFORMATICS, especially because I have to study FREE PASCAL. If I had a wish that would come true, I wish that I could reverse time to change my decision. At that time, I would specialize in Literature.