It has been 13 years to begisince I began as an accountingant but I still feel stress when I treatbring invoices to accounting software.

When you are talking about an action in the past that you have started (but are still doing), you want to use 'since I began' (begin in past tense). Also, because this is a profession (accountant) you want to use it as a noun rather than a verb (accounting). Treat would not work here, you BRING invoices to the accounting software.

The most difficult part of being an accountant is that I have to prepare draft tax returns at least 1 week before the deadline, as my colleague should have time to check transactions I made.

So I do my best in order to make itsure to draft tax returns at least 10 days before the deadline.

make sure is when you confirm or check that things are the way they should be, ie you're taking some sort of action

This way I may have time of 3hree days to check my own transactions before handing over invoice folder to my colleague who checks me.

you don't need to specify "may have time' because you already specify that it's three days. also in English, we write out numbers one-nine.

If I check my invoice transactions one more time, she canwill find less mistakes or no mistakes in the invoice folder.

She WILL find. Also missing a preposition before "invoice folder" such as MY invoice folder or THE invoice folder

It has been 13 years to begsince I started doing accounting but I still feel stress when I treasubmit invoices to accounting software.

The most difficult part of being an accountant is that I have to prepare draft tax returns at least 1 week before the deadline, as my colleague shouldneeds to have time to check the transactions I made.

This way I maycan have time of 3 days to check my own transactions before handing over the invoice folder to my colleague who checks mey work.

"time of" is unnecessary when specifying an interval like this

If I check my invoice transactions one more time, she can find lessfewer mistakes or no mistakes in the invoice folder.

Since "mistakes" are countable, "fewer" is more appropriate than "less"