Welcome to the board Rosalia.
You're certainly not alone,everyones experience of being poly is different,I am mono but have a poly partner and we discuss everything,communication really is the key. What you say to each other comes down to your own comfort levels and way of communicating on the subject. some people are an open book,others are not,its something you have to learn and feel your way with.
You needed to tell her what was going on in your head,how did she react? If it was adversely,is this the reason you feel you can't open up more?