I've had the same problem. I have a hard time knowing when I'm in love. People tell me you just know... but you have received some good advice to help you decide.
I ended up wasting a lot of time in a 9 year relationship... never knowing for sure if I wanted to marry him. I finally left him. He got married less then 2 years later to someone else.
I have met someone else who I thought I wanted to marry after just a few months. Well it took him almost a year to tell me you loved me... when he finally did it meant a lot because I knew he wouldn't say it to me until he felt it.
Sometimes I doubt myself still. Love is a strange thing. It sounds like you are very level headed and have a wonderful relationship.