You're going to have some good days and some bad ones. The trick is to turn the bad ones into useful learning points (thereby turning them into good days!). As you progress you're sometimes going to feel that you're going backwards. This is ok - it could be that your expectations of your abilities are increasing slightly faster than the rate at which the skills are becoming embedded. Don't worry about this - it just means that you're learning! Don't worry about flying with different instructors either - they're not ogres who're trying to catch you out. They're genuinely there to help you progress and share their knowledge and experience. The CFI is no different - they're just likely to have even more experience and so may be able to help take you that little further outside of your personal comfort zone (or put a little bit more pressure on by throwing in the odd extra curve ball just to see how you handle things). If you feel a bit nervous when you're flying with an instructor and that forces you to think about how you're flying and what options you have if an 'eventuality' occurs ("what if they do this to me...") then keep that thought and take it with you when you're solo.