Man oh man am I ever in your shoes. Only I have been with my boyfriend for 5 years now. You've pretty much just described my life in a nutshell, and I feel for you. No, I don't think you're being overly sensitive. I think he was hurt terribly by his first marriage, and that's still a major sticking point with him. My boyfriend said something to me when we've talked about marriage (why I want it and he sometimes does and sometimes doesn't), that showed me how disillusioned he is about the idea of being married. His marriage ended when his first wife cheated on him (this was his high school sweetheart), and he basically has lost his respect for the idea of marriage, and that it holds no permanence or symbolism for him anymore because it can so easily by undone with divorce.
He also keeps asking me what will change when we get married, and I keep telling him nothing and everything, and remind him that I now have all of the responsibilities of a wife, but none of the rights. For example, we are living in his house, and if something were to happen to him now, this house would go to his family, not me, and I wouldn't put it past them to kick me out and put it up for sale. I totally agree with the suggestion of bringing it up one more time, letting him know where you stand one more time, and giving him some space to sit on it for a while. It's a suggestion I'm going to take myself, along with the realization that if mine doesn't come around I'm looking at ending a very long relationship soon. Scary thought, I know, but much better than living in limbo.
I know my message isn't a lot of help, but hopefully it's some support to let you know you're not the only one dealing with this right now. I hope he comes around for you.
Good luck,
E.