"Questi erano gli anni in cui tu ci avevi lasciata."
Yes; if the person you were speaking to had been male it'd have been lasciato.
I am confused by this discussion. I thought because 'lasciata' uses the auxiliary 'avere', it would be agreeing with the preceding object 'ci', so it is the 'us' that were left that are feminine. I thought only past participles using the auxiliary 'essere' agreed in number and gender with the subject of the verb, in this case 'tu', the person who did the leaving, just like in French. Help! :)