Subject

Comment: Seems like you've had a rough time learning English. But I hope you won't give up. Excellent effort! ^_^

By paragraph:

l learned English for a long time. I always got low marks when I was in secondary school. Though I used to spend most of my time on English, my English skill were not improving. And my teachers occasionally got mad at me because of my poor English skills. That made me anxious about English class in junior high (･᷄ὢ･᷅). On the other hand, I think chemistry and physics are easier for me. I would be excited if I worked out a difficult problem.

Sorry but I am using an iPad, and can't figure out how to use the highlighting function of the correction editor.

Sentence by sentence: 

I is always capitalised.

It's very unusual and awkward to make a contraction with I and forms of the word learn.

While learnt is technically a form of the past tense of learn, it is rarely used. It doesn't seem to fit in this context. Instead most English speakers would use learned. 

In your sentence structure, because it is in the past tense, you would say have learned. 

You can contact I and have into I've which is probably what you meant to do.

The sentence structure ~ is the subject I've learned sounds awkward when followed by for a long time.

You might say: I've learned English for a long time. 

But i always got low marks when i was in secondary school.

We try to avoid beginning sentences with articles such as "But" and "And" there are two solutions people tent to use for this. Simply omit (leave out) the participle and begin your sentence with the next word. Or use a transitional word that is not a participale such as; except, although, or however. 

Once again I should always be capitalised.

*Note: In speech, we often ignore the rule to not begin a sentence with a participle, so you will often hear people break this rule. Also it is common to break it in casual written language such as comments on social media or in letters to friends, but never in a letter to a boss, never in school paperwork, and basically never when proper or formal English should be used. Also, the failure to capitalise the word i is common but mostly in casual typing. In casual handwriting it's not as common, and can make you appear uneducated.

You may say: I always got low marks when I was in secondary school.

Though i used to spend most of my time on english,my english skill couldn't enhance.

This sentence is technically correct (except for capitalisation and spacing which I think you understand by now.) but the word choice of 'enhance' is a bit awkward. Most English speakers would choose something else... improve is the word I'd use. Also couldn't seems too strong, I'd choose were not or weren't.

So I'd say: Though I used to spend most of my time on English, my English skill were not improving.

And my teachers occasionally got mad at me because of my poor english skill.

You have the same as problem as stated above, English is always capitalised. We capitalise the names of people, the names of countries, and the names of languages. It is considered a form of respect. We also capitalise the word God and the names of certain things such as the names of ships and the titles of books and movies, but sometimes when the book or movie title or ship name does not have capitalised letters as the author wrote it, you may use the case matching the title or name. Most people do not seem to follow the rule for the names of ships and titles of books and movies consistently thought. At least not in America. We do however always capitalise the nemsa of places and languages and people. 

Also as in above examples, we avoid starting sentences with participles. 

Skill should be skills (plural) because it takes more than one skill to learn and use a language.

Perhaps you could say: My teachers occasionally got mad at me because of my poor english skills.

Continued in my next post

Subject

That made me feared about english class in junior school (･᷄ὢ･᷅)oppositely，i think chemistry and physics are easier for me.

You chose the wrong form of the word fear for this context. It should be afraid. I do not know the rule for this, I just know it from being a native speaker. But I feel like you mean anxious.

The word English is always capitalised.

While technically it's junior high school, we typically say junior high or middle school.  

The word choice "oppositely" is so awkward, that I am tempted to say it's incorrect, but I am unsure. I would say conversely, but I tend to use more antiquated words, I think most English speakers would use 'on the other hand'.

I will simply show capitalisation in my corrections form here on out, but school subjects (class titles) are typically capitalised as well.

This should be two sentences.

One way you could write this, might be: That made me anxious about English class in junior high (･᷄ὢ･᷅). On the other hand, I think chemistry and physics are easier for me.

i would be excited if i work out a difficult problem

Work should be in past tense.

You are missing the punctuation mark at the end.

You could say: I would be excited if I worked out a difficult problem.