English diary

This months i I am changed my job schedule. I i usually work at night, but because my job partner has changed a his schedule I changed mine too (or you could say, "so did I"). so i do it .
Today is my first day (confusing, do you mean first day of working, or your first day of work since you changed your job?), of for working. i I felt feel asleepy sleepy around 1 am, so I i went outside on of my company to breathe then i breathe new ("new air" sounds strange, usually Americans say "fresh air") fresh air. Tthen I came back.
Most workers usually do not work on Saturdays, does not work usually but i I usually work on Ssaturdays. (it would be more natural to say, "Most workers usually do not work on Saturdays, but I usually do," Instead of, "Most workers usually do not work on Saturdays, but I usually work on Saturdays." The second one is more repetitive, and the first one sounds better) Ssometimes I i do not want to work on Saturdays but I have a lot of work to do from Monday to Saturday.
I select my jobs so I do not want complaint,(complaint is a noun, but complain is a verb, and you have to use a verb in this sentence) but it is my diray diary so I can write whatever I want to. Hahaha. 
Nowadays I feel that my English skills are is going to down level, so and I am worried about it. ,
When I speak English with a foreigner, I usually talk a in normal sentences, but besides I cound could not understand English sendtence sentence even a little bit. little bit little bit OMG OMG, now my i am English level is stagnant. stagnation . (this is a bit confusing...did you want to say that your English level is staying that same, because that's what stagnant means, or did you want to say your English level is getting worse?) OMG OMG
Sometimes I can understand what is on BBC News and CNN. 100% i wish that but when ??? (what are you trying to say? You wish that you can 100% understand BBC News and CNN? Or that you already 100% understand BBC News and CNN?) ㅠㅠ

You will always improve your English by writing notebook entries and talking to foreigners! Don't be frustrated or sad! You are doing very well already! ^_^ Make sure to not leave spaces after periods because it will make you seem more like a native speaker. Also, don't forget to capitalize words like: "I" and "E" in English, and days of the week (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday). I hope I helped you! 화이팅!

English diary

This months (month is not plural) i I (always capitlize I) am changed (You have to match the tense so either have changed or am changing) my job schedule i I (always capitlize I) usually work at night because my job partner has changed a schedule so i I (always capitlize I)do (again, you need to match the tense. use Did for past tense, Am doing for present.) it too.
Today is my first day , for (when) working i I (always capitlize I) feel asleepy around 1 am , i I (always capitlize I) went out on my company Outside then i I (always capitlize I) breathed new (word not needed) fresh air then came back .
Most workers on saturday does not work usually (word not needed) but i usually work on saturday , sometimes i do not want to work on saturday but i have a lot of work from monday to saturday .
I select my jobs so i do not want complaint but it is my diray so i write hahaha .
Nowdays i feel my english skills is (are, "is" is singular) going to (word not needed) down level (word not needed) so i am worried about it ,
when i speak english with a foreigner , i usually talk a normal sentences besides i cound not understand english sendtence little bit little bit OMG OMG , now i am english stagnation . OMG OMG
Sometimes i can understand on BBC news and CNN 100% i wish that but when ??? ㅠㅠ

Your English is quite good, need to work on capitilization and which tenses to use. (I didn't fix all of the capitilization errors because the above post looks like it did that already)

I think if you just practiced talking with a native speaker for a little while you will be very fluent soon.

good luck :)