What are the measures to keep your private key safe and in memory ?
Print your private keys to any form of paper that you can keep for safety. Don't trust your memory even you are the champion in memory verse challenges, it isn't enough to keep your funds.
You can also create a file on a notepad and save it to a flash drive and don't use that drive to any computer that you don't own. Don't use it to public places.
Are there any alternatives to retrieve the key back ?
If you lost it, that's lost forever.