don't worry too much about messing up your pics. i use to be the same way. i'd draw for hours only to ruin them with a coffee spill or something. just keep in mind that if you could do it once you can do it again. sometimes they come out better after a re-draw. and nothing lasts forever, i'v found that the act of drawing far excedes the feeling of the finished piece. maybee that "oops" youve done on the piece actually works with the piece. if all fails get some sleep.
Ok i love drawing, i love sketching i love art! But because i do so much i expect so much i sit there for hours and hours drawing carefully details and then either i muck up or smudge it or get knocked i feel like crying! Does this ever happen to you???