If I decide he's not the one how do I know I won't meet somebody else, think its all perfect and then have the same thing happen again?
Hello OP, been there, done that. I'd been with my then DP for 4 years, living with him for a year, and he proposed marriage. I said yes, then had second thoughts and backed out. We stayed together but the doubts and anxiety drove me to a nervous breakdown and I was quite ill for a few months. It was really tough on him. Why did I feel like that? Well, long story, but the short version is that deep down I felt I didn't love him like I thought he deserved to be loved, that somehow I wasn't good enough for this person who I knew loved me completely and would never let me down. But I was just as terrified to walk away because I thought, if I meet someone else and fall in love with them, how do I know I won't get the same doubts again?
This is the key, I think, to realising that you do love him, that he is the right one, and that the doubts are all in your own head arising from whatever lies at the root of your tendency to anxiety.
It has already been said that there are no guarantees in life - you can't know for certain that if you commit to this man your future will be happy forever. Nobody knows what stresses life will present you with and how well you will both cope with them - all you can know is that you'll do your best and that you believe your DP will do his best too. With my DP I tended to think in absolutes - marrying him needed to mean 100% certainty and would result in 100% happiness. That was just too scary however - and with good reason: who is capable of 100% certainty? - that's a goal I was never going to achieve - setting myself up for a fall - and 100% happiness - how boring, how stifling! No wonder I used to visualise marriage as the end of all the growth and development in our relationship.
Also, it's already been said - if a decision is causing you anxiety, no decision is OK until you feel stronger. My DP said something to me that helped: "Do you want to be with me today?" "Yes." "Do you want to be with me tomorrow?" "Yes". "Then let's start with that and let the days after that take care of themselves."
Anyway, sorry, probably over-projecting a little here but hope it helps for you. My story ends happily - together 18 years and married for 9, 2 kids, normal ups and downs, no more doubts. Hope your story will end happily too.