The change that made the walk back to the window more difficult than walking away from it has everything to do with Tom's attitude.  When he goes out the window onto the ledge, he is single-minded in focus - he simply has to retrieve the paper.  He has put too much work into it, and it would take more time than he has available to recreate it.  The danger of the situation really isn't a factor that crosses his mind in any real sense.  However, when he grasps the corner of the paper, he has a visualization of his precarious position, and that is when the enormity of what he has undertaken strikes him.  It is at that moment that the true danger of his predicament becomes a harsh reality, and his focus becomes returning to the window safely.  Because he has experienced the full terror of his situation, with every calculated move he makes back to the window he feels the weight of his potential demise.