First,I want to say "thank you " （双引号里面第一个字母最好用大写）for （to）you (and wish want you to) still stay with me.
Formerly,I had many firends who had (去掉had) promiss(+ed) me to keep firendship forever.
But I find（注意时态,你总不能一直都发现这个吧）（加个that会好点） none （of them）stay with me when I go farther and farther.（整段都注意时态）
Now,I just have you.So I want to thank you again and again!
You let me know what is firend（you let me know about what friend means）,what is firendship(what friendship is/means).
You let me know,to have a good firend and have firendship are very beaytifly tings.
I don't konw,does I'm (am I)a friend of qualified in your heart?
But I'm sure (that)you are a good friend.
I think ,a good firend can help you study and encourage each other to make progrees instade bring (你不发现这里有两个谓语动词了么)trables.
A good firend can share your happiness and sadniess（sadness）.
The most important（用副词+ly）,a good firend who is always believe you.（不能成句,只有主语.你可以改成the most impotant thing is that a good friend always believes you）
And he or she will try his or her （their）best to understande （没有e） you instead find（instead of finding） your fault.
However,these you are done(被动语态?你被做了?).
You will appear to comfort me when I （am）sad;You will laugh（smile） with me when I （am）happy.You （have）stuy （ied）with me already have（去掉） four years .
although I'm not the qualities of a good firend,I'll try my best to do it well!Until like you treat me .
I hope ,our four-year（形容词） firendship can cntinue the next（more than） four years and four years .
My privilege to have you !(拥有了你?)
Dear,Don't leave me .Bcause Without your accompany,I walk really lonely.
Best wish !
说实话,初二水平……不过感情真挚!（冒昧问一下,你初几?）
加油吧!
辛苦你了,也累坏我了~