Having a LOT of trouble closing the cash register at work?

I recently got a job at a place serving food (it s a small place, so I take orders and handle cash as well as serve it), and so far after training I ve only worked two days - but both of these days I was closing, and for some reason, I CANNOT figure out how to close the cash register! The first night I was crying because I was so confused, and last night, I understood better what I was doing, but I was $20 short. I called the owner explaining this, and he said just to do what I could and he d check it out tomorrow (today). But now today a manager texted me that I "left three orders open, didn t close and reconcile, didn t put everything inside the envelope and didn t do the math at the end." I thought I did everything I was told, despite the money being short, which I thought was just an error because I KNOW I gave the right change and stuff...I just feel so stupid because I ve been trained on closing TWICE. This coming Sunday night, a close friend who also works there is going to show me again how to close, and it ll hopefully be better with her, but what the f**k? Has anybody else ever had this much trouble when first working with a cash register, or is it just me?! I m good at taking orders, serving, and helping customers, but the whole closing thing seems so hard for me. Will I get it in a few more tries, or is it just that I m horrible at math and shouldn t be doing this? I just feel so stupid. :(