I've been with my boyfriend for about 3 and a half months now and all he ever does is go out with his friends and leaves me at his house on my own? Why?

How would we know? You sound young (thinking we would know why he does what he does), and he sounds young too, because young guys often go back to hanging out with guy friends after the romance is over. Time to move out and break up. You aren't going to change him. Or anyone. You also need a life of your own so that this doesn't happen with the next guy too.