I have been in similar situations, in university, school, temporary work, holidays....and it's the reason why I have made so few friends in life that practically none are left. I can be friendly and quite talkative and outspoken in the right context, one where I feel "safe" so to speak, and once I know people at least a bit.
But the great problem is the initial approachment to strangers, because if you are very silent at the beginning you will be assumed to be "silent and boring" and this will make it harder to enter the network of friendships, especially once the bonds are formed between people. It also makes you embarassed to be perceived like that, which aggravates the problem. A vicious cycle.
I can't give you any valuable advice I'm afraid, except the obvious one to still try and slowly make yourself at least "visible" to your flatmates. You don't have to change personality, just drop a joke from time to time, try to look friendly, it may not work but there is a reasonable chance that you might break the "invisible barrier" so to speak.
Other than that, I can only confirm that we are all in the same boat, and wish you good luck!