Why is 300mg of melatonin not making me tired enough to sleep properly?

It doesn't do that: First, melatonin is not FDA controlled. Do hamburgers vary ? Melatonin is no more regulated than foods and there's zero quality control on whether you are really getting 300 mg. Then assuming its ok, it is the hormone that tells you it is dark. THis might be a trigger to sleep, but it is NOT a sedative. Unless you are tired, due for sleep, etc. It won't do much.

Sleeping Troubles: There is often a rebound effect with Melatonin. The more you take the less it works. But, your question spurs me to be very frank with you. Your sleeping troubles will not be solved by pills. At your young age you are setting yourself up for a lifetime of trouble if you are already relying on overdoses of pills to get to sleep. Perhaps it is time to get your anxiety treated in Therapy.