And, if you do, how do you achieve that?
Before every lesson I always feel that I should have my pieces at a performance level?
Bernadette, we crossed paths recently in another thread, and to some extent we've had some similar history. At the end of my first stint, going into the second with my present teacher (and switching to piano) I looked deeply into what lessons are actually about. You have a misperception, and it is probably at least in part due to your past experiences.
Lessons are (in my view) for the purpose of acquiring skills. With any teacher I engage, I want to make sure that we are both clear on that point. There are even teachers who would like to focus on skills, but are afraid to do so fully, because their students may not have that concept. A lesser form of lessons focuses on pieces, and you might have a teacher where every week you "perform" for the teacher - she says "X could sound better - listen to me play it. In Y you keep missing the F# /you're not bringing the dynamics [but I won't ever think I should teach you how]." etc. And so we get the idea that we're performing for the teacher.
With a decent teacher / the kind I like to work with - the skills are the project, my growth is the project, the piece is not the project. There is almost a delight in finding weaknesses, because this tells the teacher "Here's what we need to tackle next." That also becomes your project as a student and you practise. You are not performing; you're cooperating on this project. It is a totally different mindset.
That said (and this goes with (I think Bob's?) - answer to another question of yours. Say a teacher wants you to get a handle on playing dynamics differently in the RH than the left, for voicing. The teacher gives you a specific way of practising this. Then the teacher wants to see signs that you practised the thing he focused on with you, in the manner that he showed you. Say he wants you to play one hand loud, one hand soft, beat by beat, gradually bringing the timing together (I was given this some years ago), then when he hears you play the next time, he will hear whether you did that. It also tells him whether his idea worked with you. If you come in playing "beautifully", trying to show him the wonderful performance you cobbled together, but you did not try what he told you to try, he will be disappointed.
As dogperson said, your teacher may also want to know about problems you are having. This one is tricky, because:
a) the teacher may want to hear you play, for him to recognize problems
b) the problem you are having may be due to something different than you think. As an example, some difficulties I had with my hands "kinking" were due to where I was sitting, and where I balanced my gravity. Had I insisted on fixing the hands, rather than letting my teacher tell me, "Do this. Now do that." while observing, it would not have worked well.
All of this to say that "performing well", or playing a piece as perfectly as possible, isn't actually in there for the kind of teacher I like working with. (Does any of this make sense?)