I do this all the time too - even if it's something I think I really want/need to talk about and discuss with her, when I get in the session I will go mute/get distracted and talk about other stuff/do surface level tiptoeingaround the topic, which doesn't really get to the core of what I wanted to discuss/be otherwise avoidant.
If I repeatedly don't manage to say what I really want to say, I get really frustrated with myself and will maybe then email her to say, I want to talk to you about X but then get in the room and can't/don't say it... It's partly me venting and partly giving her a heads up.
Also, I occasionally text her on the day of our session to say 'there's something I want to talk to you about today' or 'I want to talk to you about X today, so if I look like I have nothing to say, can you please mention this?!' It's really a quick, light way for me to try to make myself accountable for saying what I need to. Obviously, if she then gives me a nudge about it when I see her and then I still don't say (which has happened before) that probably tells her something too!
As some others have said though, this is slow work and it takes time to build a relationship and to feel safe enough to go there with the more difficult stuff. So I'd also agree with not rushing it. This:
how to ask about the right things I should be asking about,
sounds to me like you're putting yourself under pressure...to do things properly, to get things done,.. The 'right' things you 'should' be asking about... I do this too - my therapist has told me countless times that there are no right or wrong ways to do therapy and that I can just show up and be however I need to be in any given session. It used to drive me nuts because I wanted her to tell me how to do it right and what I needed to do to make progress most quickly so her patient smile and 'this will take time' message used to frustrate the hell out of me. Now - I try to be accepting of the fact that how I am in a session (whether that's chatting about nothing in particular, being avoidant, trying to entertain her with funny stories, dissociating) is for a reason. So I try (don't always succeed!) not to give myself a hard time about it and not to be in such a hurry to try to forcibly move things on.
Also agree with those who suggested 'talking about talking about it.' It could open things up a little and allow you to continue to build your relationship with your therapist so that you can dip your toe a little and see if you want to start going there with her at the moment.