Your hotel should give a receipt to you if you ask.
Maybe just ask them to file it again and get a fresh receipt.
If they are not filing your TM30 registration, they are not following the laws.
The consensus seems to be that it's easier to just head out of country and come back in though. Going to Immigration to extend requires much of the same, if not all of the same documents as you applied with, and some need to be more recent and up to date.
And you will need 500,000 baht or equivalent in your countries currency, saved in a bank account.
The applications itself costs around 10-15,000 baht depending on where you apply. With some places being far more expensive ( New Zealand, looking at you! )
Good news:
You're only 19 and can spend time saving up for that goal, while you work on getting a higher income per month. $400 won't get you much here.