I don't care what anyone says, I have always believed that being married is different than being "in a committed relationship." (I shouldn't be talking, because I got married in Vegas, but I do take marriage so seriously!) Unless you view marriage as simple as a legal document, it provides a different feeling of security and partnership and maybe after almost five years with this guy and no marriage proposal, you are questioning his devotion to you. Anyway, maybe this is part of the reason you are feeling anxiety. I hope your boyfriend is sensitive to that. I don't have any advice but I wish the best for you.
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"Go either very cheap or very expensive. It's the middle ground that is fashion nowhere." ~ Karl Lagerfeld
Don't feel bad for feeling that way. I am in your same boat except my bf has all the worries you are having. We met our freshman year of college and are coming up the the 5 year mark. One of my early rules though in the relationship was that I refused to even get engaged until we were both out of school with degrees and a decent job (no working at circuit city or radio shack type jobs) and unfortunately he is taking the long route in school and I went back for grad school and the waiting game gets tired but everything is going to work out because we do love each other.
I don't know what your current situation is or why you put off getting engaged but I'm sure its something yall talked over and he understands. He probably gets emotional like you do and wonders the same thing like does she love me, does she want to stay with me forever and get married, etc. because I get those feelings and its my bf who worries over it more like you do than I. If you've been with him for 4.5 years I don't think he's going to really wake up tomorrow and not want you. That's just too long to string someone along and that is what I tell my bf when he starts getting worried/insecure. Luckily we are both graduating this coming up year so hopefully something will happen and all the worrying will end.