I have both of these problems! It's taken me 20 months of weekly sessions to come close to trusting my counsellor, but I still find it hard to open up and when I do, I'm convinced that I haven't explained myself properly.
My counsellor clarifies things with me though, as he knows I worry about being understood, and he reads between the lines more often than not, so they generally understand what you mean, even if you weren't clear.
Mim