I think this happens to all of us when we go into a shop if we don't have a specific project in mind. I find it far easier to start with a specific pattern and then shop. Then I know how many different fabrics I need; if I need any coordinating or contrasting fabrics, etc. Otherwise, yes, you probably want one of everything. That said, I'm not a stash-buyer. Don't have room for a stash plus I know my tastes will change over time. And/or I'd not have enough of a specific fabric for the project in mind at the time. I do over-buy by anywhere from 1/4 to 1 yard of any given fabric at the time. Gives me room for booboos and a small stash for putting together the odd block for something, trying a new technique or whatever.
Just take your time, you'll get more comfortable with the whole process.