I'm sorry for your loss, I'm pretty sure the hacker was waiting for you to receive any funds to step up and steal your BTC.
~snip~
Anyone has any idea of how this can happen? Each time I transfer, I need to insert my password in Electrum.
As already mentioned here in the Topic, it's likely that your PC is infected with malware that allowed the hacker to access all of your data. This means that it's not just a matter of your password being compromised, even if the hacker doesn't know your password, they may have been able to obtain your seed phrase if you stored it somewhere online or on your PC, which would allow them to easily access your wallet by importing your seed into their own Electrum wallet.
Long story short, it's important to never store your seed phrase on any online device. Using an air-gapped device or a hardware wallet is the safest option to ensure the security of your wallet and protect it from hacking attempts.
Any technical glitch or my money is really gone forever?
Panicking here
Once a transaction has received one confirmation, the money is typically considered irretrievable unless the hacker chooses to return the funds, which is highly unlikely since I never seen that happened before, so sadly to say yeah, your money is gone forever.
In addition to that, there have been no reported technical issues with Electrum in terms of wallet security. Therefore, it's highly unlikely that the problem lies with Electrum itself. It's more likely that the issue is due to inadequate security practices on your part.