I know there are quite similar questions, but none have addressed my issue. Let's say I use Electrum wallet, that generates addresses & keypairs from seed. So for every next transaction I use different private key, public key & address. So (as I understand) when I want to buy something I send my address encrypted with private key to recipient, he decrypts it with public key & then checks the blockchain to determine if I can afford the transaction. But since I use different address for every transaction, how is that possible? I don't see way to connect current transaction to all that came before.
So (as I understand) when I want to buy something I send my address encrypted with private key to recipient, he decrypts it with public key & then checks the blockchain to determine if I can afford the transaction
Not quite the way it works. You just send the address, which is essentially the public key, to the person who is trying to do the paying. Given the address (more technically, references to times that address has been paid to on the blockchain before), anyone (not just the payee/payer) can verify that the transaction has the funds to spend.
And rather than sending the private key to show that you are the owner of the coins, you send a Digital Signature, which you need the private key to generate.
Your wallet reports your balance by looking at the list of times that you have been paid to before, which you have also not spent. This will be across many addresses. The network doesn't know which payments on the blockchain are yours, you have to reference a few of them when you want to spend funds, and prove you own the key to them with a digital signature.