给我点建议？？

我作accounting at small company. I do A/P. We allocate cost into each dept. 有时， 我分错了dept around $20因为Fedex put名字不对，My boss要说。invoice 总数doesn't match PO, My boss also blamed me and asked me if I asked for Engineers who generated PO. Also asked me to redo expense reports due to no Canadian $ conversion to US $ -$4 difference.
I wonder if I am too careless. Or the boss is too picky.
BTW, I have tons of things to do...A/R, payroll, Month End and misc. stuff~~~
Everyday I want to leave but I like the company. Pepole here are nice.
Just Overwhemling with workload and my boss.
Pls advice!