You're impossible to forget.
sei im-pos-SI-bi-le da di-men-ti-CA-re — stress on 'si-', 'ca-'. 'Impossibile' has five syllables: im-pos-SI-bi-le.
After a first meeting or brief encounter — expressing that this person has left a lasting impression and you cannot stop thinking about them.
'Sei impossibile da' + infinitive = you are impossible to... This construction is very common in Italian for expressing an overwhelming quality. 'Dimenticare' = to forget. The phrase says the impression is indelible.
Non riesco a smettere di pensare a te.
I can't stop thinking about you.
Direct — the mental preoccupation is explicit and flattering
Hai lasciato un segno.
You've left a mark.
'Lasciare un segno' — leaving a mark — is vivid and permanent
Ti porto nella testa ovunque vada.
I carry you in my head wherever I go.
Poetic — the person becomes a mental companion in daily life
The idea of being 'indimenticabile' (unforgettable) is a powerful one in Italian culture. Italian songs and films are full of encounters that leave permanent traces on the soul. Telling someone they are impossible to forget says the meeting has already changed something in you.