how many hours have you had?
pretty much everything you have said are just things that come with experience and practice, when you're learning of course you'll need to consciously think about what you're doing, its part of the learning process, its the needing to think that leads to doubt, I wouldn't worry about it, everybody goes through the same stages, but one day it'll just click and you'll just drive.
the approaching junctions and roundabouts, its just about forward planning, making a decision and choosing the appropriate gear etc, it'll all just come with practice, soon you wont even have to think about what you're doing.
If you slow down a little earlier, you'll give yourself a little bit more time to make the decision and get yourself sorted out, what it all boils down to, other than the mirrors, signal etc etc is whether you'll be going or stopping and taking the appropriate action, the suddenly stopping comes down to not having made the decision about what you're going to do, or making the decision but not allowing enough time to get yourself sorted out, and your brain goes bleugh, and you stop. so give yourself a bit more time, or be more decisive, you could vocalise what you're planning on doing, it can help with going through whatever routine you do.
The key is to not over think what needs to be done, in theory its easy, driving is childs play once you've gone through the 'learning' part, in practice its a little harder because when learning you need to feel comfortable with what you're doing, making the right decisions, being confident in your decisions etc and all of that just comes with practice.
I'm sure people could reel off some spiel saying do xyz, do this, then that whilst doing the the other, but it wont really help, im sure you know what you need to be doing, you just need to get comfortable with your abilities as a driver, it all just takes time.
The harsh breaking, when you're about to stop, come off the brakes slightly and reapply them, it'll take off that harsh feeling jolt at the end.
The whole changing instructors is a difficult one, sometimes its a case of better the devil you know, there's so many variables, but to me it just sounds like you're at the stage of feeling cr@p about your abilities, a thought im sure the majority of people have been through, thinking you're not moving forwards, when in reality you probably are, and are getting better with each lesson, but just don't realise it, because not much actual new learning is going on.
It might be worth booking an assessment lesson with another instructor, might give you a better understanding of where you're at.