Maybe to solve that midwicket problem try to keep the ball on the ground throughout most of the innings until you get into the "zone" were you are on about 20 or 30 but your timing the ball well and your feeling confident and settled in. To build up your score to 20 play all the full balls straight and let the wide and short ones go. In playing straight if your driving the ball well and hard but straight to the fieldsman, block the ball into cover and run a quick single. Go all the way forward and play it with confidence. Often when im batting because i go so forward my block has enough power to go past the fielder at cover. Anyway if youre scoring your runs at the begining by blocking or defending the ball into cover they are likely to put a fielder in close so that may create more gaps in the field especially if theve put cover closer because if you're confident enough and you go really forward to the ball in your drives your always going to make runs. Getting caught at midwicket, is that from crossbat shots? Try to avoid playing shots like that unless you are completely settled in and the field is up.

This may be a mental issue, your concentration may be dropping off when you get to 20 or 30. I suggest you work on this aspect of your game and build a game plan/routine as to how you will approach your innings, what you will do between facing each ball, etc. I suggest you look up Rudi Webster's book "Winning ways : in search of your best performance", a renown sport psychologist, who focuses on cricket related matters.