I'm writing a story where the main characters are Italian and need help translating a few phrases. I tried using translation services online but I got different answers.

'our eternal secret' (like something was a secret forever between two people) I got segreto eterno/eterna & segreti eterni for this one. Are any of these right?

'you're welcome' (to a male)

'I love you' (to a female)

'thank you very much' (to a male) I was going to use 'Grazie di cuore' if that's correct.

Thank you for your answers!