Money just flows out... I learn a new lesson every other day...?

You haven't learned the lesson bud. The lesson is that everyone will always ask you for handouts for one thing or another, and if you truly want to help them, you will say no. People who take money from you have a much larger issue than not having money, it's that they themselves don't know how to manage their own money, and giving them more of it doesn't help them solve their problems.

But if you ever do want to give money to someone, then you treat it as a gift you can afford to give them and never see again. If they return the money, they value the relationship they have with you, and if they don't, then they're not exactly good to keep around in your life.

As for the shop-keeper, they're lazy and obsessed with making sure that the line goes through quickly so that they don't get any weird looks for them having screwed up. Be nice and firm that you want change recounted, or whatever it is. Go through it with them if they're just trying to get you through as quick as possible, they'll feel embarrassed and apologize, but by saying no worries, you make the same mistakes, they'll feel a lot less embarrassed and more inclined to make sure they're doing the till right for the next customer.

As for the receipts, you should be getting into the habit of keeping all of your receipts so that at the end of the day you can tally up how much you spent that day and also compare it with your bank account statements to ensure that everything is correct. You'll also find yourself spending less money on things you don't need because you'll feel guilty about having spent $5 on a coffee every morning (hey, that adds up to $1780 a year that's fueling your brain on a spike for a couple hours every day). Pay attention, it's your livelihood that you're letting go to waste when you're not careful with your money.