Well, seriously, sometimes our words might offence their feeling.

I suppose if i were she, i will felt that, " Why my boyfriend will simply think of that ? Don't he know that im having exam recently, i have lots of subjects need to cover up, why he don't understand me at all ? Why ? He dont even care about me !! T.T "

Assuming if my statement is correct, then i have a suggestion to you, give it a try.

First of all, for your information, girls like guy to be honest to them and let them know exactly what are you thinking. Because sometimes, they are kinda tired on guessing what their partner thinking. So, the interaction between both of you must be good.

Find one day in which she 's not so busy and kinda free, ask her out to somewhere peace and provide good atmosphere with nice food and music. Recommended : Starbuck, coffee bean and etc.

Order a piece of cake and coffee. Sit down, speak softly but seriously. " Seriously " is to let her know you care about the relationship between both of you. Speak out the reason why you said to her like that in the previous conversation. Tell her, you are sorry for that incident, but you really still care about her alot alot alot. ( Your expression must be good and honest enough, dont smile and laugh, she will think that you are not taking it seriously.)

After that, tell her, you wish to listen to her words too. Im sure, she will definitely vomit out all the words that she kept so long in her heart. Well, at that moment, i believe you will listen to lots of issues in which it might be normal and nothing special to you, but meant alot to her.

At last but not least, of course must end with something romantic. At first, Prepare a bonquet of roses and ask the counter to keep them for you. Present your roses to her and say you love her very much.
Roses + chocolate drink + cheese cake = 100% make her sweet deeeeeeeeeeeeeeeply in her heart.

Well, how to maintain the relationship btw both of you, its all up to you. 
All the best ! From your message, i know you love her. exactly the same i love my gf.

Regards,
Perfect-Stranger