You are depending on him for your happiness way too much. If spending the day with him makes you feel on top of the world but being without him makes you come crashing down you are not experiencing healthy emotional states at all. And chances are he is picking up on your clinginess which is potentially driving him further away. I know you say you find it hard to talk to him however if you want this issue resolved you are going to have to discuss it with him. Find out if his studies are the sole reason he can not see you as much or if something deeper is going on. Until you do you will most likely remain in these states of highs and lows.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!