Dominion remodel card -can I trash a card already played?
No, you cannot do this. Your "hand" always refers to cards that you physically keep in your hand. Cards that you have played this turn are in a different place, the "in-play" area.

On page 6 of the rulebook:


  To play an Action, the player takes an Action card from his hand
  and lays it face-up in his play area.


Pages 9 and 10 also show picture examples of the different places cards might be; including hand, deck, discard, or in-play.