There is no sin on a person who drops the Mus-haf unintentionally
Question

I am 18 years of age now, when I was only 10 I was reciting the Holy Quran once and then got up to offer Salah, so I quickly went and kept the Quran inside the cupboard on top of a box. After I finished offering my prayers I realized alas that the Holy Quran had dropped to the floor! I asked my Lord for forgiveness and I still ask His forgiveness after every Salah that I offer. Still, I am not at peace and want to offer my repentance once and for all! What should I do?
Praise be to Allah.No Muslim doubts that it is obligatory to respect the Book of Allah, and the scholars are agreed that the one who deliberately mishandles it is a kaafir. 
At the same time, Allah says that there is no sin on one who is ignorant, forgetful or makes a mistake. 
Allah describes the believers as saying (interpretation of the meaning):
“ ‘Our Lord! Punish us not if we forget or fall into error’”[al-Baqarah 2:286]
According to a report narrated by Muslim from the hadeeth of Abu Hurayrah (125), Allah said. “Yes”; according to another report from the hadeeth of Ibn ‘Abbaas (126), Allah said, “I have done that” (i.e., responded to the prayer uttered in this aayah).
Whoever has no free will in a matter, such as one who is forced to do something, or one who is asleep, is not sinning if he says or does something that goes against Islam. 
You have not done anything that goes against Islam. When the Mus-haf fell it was not by any deliberate action on your part, so you have not fallen short in respecting it. Although Islam tells us that we must repent from our sins and give them up, at the same time it warns us against falling prey to waswaas (insinuating whispers of the Shaytaan) and despairing of the forgiveness and mercy of Allah. 
We ask Allah to help you to do that which He loves and is pleased with. And Allah knows best.