I tried to use Electrum but the receiving address is not the same as the sending address.
Is there any software or app or a website that provides you with a Bitcoin wallet - where your Bitcoin Address remains the same for both sending and receiving without any changes whatsoever?
You can only send funds from an address which you received BTC to earlier.
And address stay valid
forever.
So.. you can always receive BTC to the same address. And if you only receive funds to this address, you only can send BTC from exactly this address.
Even if you receive funds to a different address, you can choose the inputs to consume from the 'coins' tab in electrum.
Wallets do only create additional addresses for you. You don't have to use them.
You can either only use one private key in a wallet or simply create 'pay-to-many' transaction spending the whole balance (one part you want to spend, the rest back to the same address).