I'm coming back(≧3≦)

Dear friends, I haven't seen you for/in a long time; I miss you all so much.

We normally include the subject when we start a composition. I think that "in" sounds better than "for" in this instance, but that's just a preferance. You're not wrong. The semicolon (;) is because I miss you all so much is technically it's own sentence but links in with the previous sentence. It's just pedantic conventions. I added "all" because you are talking about multiple friends.

I am very nervous today because our TEM4 scores will be revealed╮(╯_╰)╭

I changed made scores plural because all of the scores will be revealed. Again this is a pedantic correction, nothing to be worried about.

I chrish the hope that I can passed the exam. This exam is of great importantance for to me (≧∇≦) If I failed this test I will would feel extremely sad!!!

Cherish the hope should just be hope. Would versus will is extremely complex to explain using grammar rules, so I won't put it here.

I'll would have to take this exam again in the next year. But what would affect me most is that because I'm now a junior student in college, I have so many things to prepare for, like my postgraduate examination, my teaching qualification certificate etc. I don't want to waste time on this test ever again! I'm really, really nervous. God bless me *^O^*Let me pass ~~

We just say junior in college, or it's my junior year in college. If you need anymore explanation for corrections feel free to ask me. Good job using affect instead of effect, I can't tell you how many natives mix that up! 加油，你的英文这么好你肯定学了好久！那么久的用功学习我相信你的成绩一定会很好！

I'm coming back(≧3≦)

Dear friends,_haven't seen you for a long time,_I miss you so much._I am was very nervous today because our TEM4 score will be revealed╮(╯_╰)╭_I chrish the hope that I can pass the exam. This exam is of great importantance for me (≧∇≦)_If I failed this test,_I will would feel extremely sad!!!_I'll have to take this exam in thenext year again._But what affect me most is that I'm now a junior student in college so I have so many things to prepare like my postgraduate examination,_teaching qualification certificate etc._I don't want to waste time on this test ever again._I'm really really nervous._God bless me *^O^*Let me pass ~~

_ = Space