I worked a few times for free until the client decided he doesn't need anymore that product so he doesn't need to pay me.
From that moment i started to ask for advance payments, but a lot of times I calculated incorrectly the real price of my work miscalculating the number of hours it will take, ending working for less than 3€ per hour...

I love freelancing, but I'm really bad at evaluating my work.

Get a contract and a lawyer. Charge by the hour.

@projektaquarius I'm still a student, and do some freelance small projects in my free time, can't afford a lawyer.

Find out a logic $/h ratio
Make a weekly summary and one monthly
Make a contract, which says, that your contractor has to sign a copy of your report and billing based on this report.

Therefore, you are safe, when it comes to after hours, you and your customer have full transparency on your work time and your costs.

They have to accept your billing, which gives you additional safety...
And yeah, a lawyer ensurance is well spent money.

Upwork

Just multiply your estimate by 10.