Why does it suddenly and randomly get worse?

Why does it suddenly and randomly get worse? I'm a 13 year old guy and I've stuttered as long as I can remember. In the last few months it's gotten way, way worse and I don't know why. It's made me really, really self conscious. I don't feel like I can speak at all anymore. There's a girl I really like and we used to be friends but I have barely talked to her in a couple months because I'm so embarrassed. (Her family was over to play games last weekend and I pretended I had to work on an assignment and stayed up in my room almost the whole time.) It's not like she doesn't know I stutter because I've always stuttered and she never made a big deal about it, but now I can hardly get anything out and it takes me so long and I sound like a complete idiot. Plus I make weird faces and I've started sort of ducking my head a little bit when I try to speak and now I'm having trouble stopping that also. I look so retarded. I asked to get out of all presentations and things like that and I was allowed to for the most part. But I feel like I'm failing at life and everything else because I actually really want to do all those normal things, and I feel guilty to get out of them, even though I'm also glad because it would be a nightmare. I don't know why it's gotten so much worse and what I can do about it. I try to get enough sleep and I wasn't all that stressed out until my stutter got so much worse. Two of my younger brothers also stutter, but it's more mild like mine used to be, my dad stuttered only when he was a kid, and my grandfather has a stutter too and his is still pretty bad. I live next door to my grandparents and so I work with my granddad a lot. He's the only one I feel like I can really talk to about this, but now I feel like he's ganging up with my dad to try to convince me to do speech therapy again. Speech therapy just made me feel dumb, and also really guilty that I wasn't improving, but my granddad wants me to try speech therapy again with a better speech therapist. Is it bad that I don't want to try it again? It didn't help at all last time and my stutter wasn't even as bad at that point.