Mary,
I've felt exactly this way before. Your words could have been taken right out of one of my diaries! I encourage you to do your best in what you are given to accomplish each day (studies, job, chores, whatever), and let God take care of the rest. Trust that He has a divine plan for your life that will far exceed your expectations if you will but obey Him and follow Him. Look for others who need your help and lend your hand willingly. Look to bless others. Don't neglect daily time in the Bible and prayer. This is your strength and the Truth that will combat all the lies the enemy feeds you which bring you to a state of discontent and unhappiness and self-focus.
Heavenly Father, Help Mary. Minister to her today and help her to realize how precious she is to You, how much You love her, how much You have protected and provided for her, and how much more you have in store for her, if she will follow You and dedicate her life to pleasing you. Help her to identify and remove all negative influences (TV, Fashion magazines, worldly movies, fantasy, etc) and to fill her mind with all that is lovely and pure and noble and pleasing to You. In Jesus' Name, Amen.
(Philippians 4:8) Finally, brothers, whatever things are true, whatever things are honorable, whatever things are just, whatever things are pure, whatever things are lovely, whatever things are of good report; if there is any virtue, and if there is any praise, think on these things.