Have you ever heard someone say something to the effect of "I am only striving to do God's will."  Well, if we are really following God's plan for our life, doing His will should never cause us to strive.  He has promised that whatever He calls us to do, He will empower us to do.  So, if we find that we are struggling,then we have begun to rely on our own strength instead of God's empowerment.  Sometimes we may struggle because we cannot believe that God will do what He promises us He will do.  When we see the mountain, the mustard seed seems awfully small.  Yet, God promises us that a faith as small as that mustard seed will move that mountain if necessary to accomplish His will.  We just have to make sure that we are trying to move the mountain to accomplish His will and not our own.  It really is quite simple, either we believe God or we don't.  He will never force us to believe.