What happens if someone sends me more bitcoins than I set in my public key?
If you are talking about if someone send more than the amount you requested for using the generated QR then you will receive all the funds to your wallet.
How can I create a public key that lasts forever, that I can get receive all differents values?
It depends on the wallet you use, some wallets changes your public key very often and some lets you use only one specific address.