Something's wrong with me

I can't say no. I just let things happen even when I don't want them to. I don't know why. I mean, I have said no in the past if a guy wanted to do something way out of my comfort zone, but most of the time, I just let them do whatever they want and try to pretend I'm enjoying it. Do you know how difficult that is to explain to someone? It's like there's a disconnect between my brain and my body. On the inside ever fiber of my being can be screaming "NO" but all that comes out is, "um...yeah, sure." Maybe I'm just very weak. Is there anyone who can relate to this? Nobody in real life understands me when I try to articulate this and it makes me feel crazy or like I'm just making lame excuses to myself.