Let me start by telling you - I have been married twice. The first time should NEVER have happened. I was running away from a bad home situation and got myself into a bad marriage situation. The only good thing that came from my first marriage was my wonderful son.
Now - with that out of the way - I have been in love with two people in my life (the person mentioned above not included). I am now married to one of them. If I had ended up married to the other one, I think we would have had a very happy life together (and I would have avoided husband #1 altogether
). We are still very good friends almost 2 decades after our relationship ended.
So, how do you know to answer "yes"? I think your heart knows. I know when John proposed to me, the actual proposal was a surprise - when, where, etc. - but I knew it was coming at some point. I was at that moment the happiest girl in the world, crying, etc. - just like in the movies.
Yes, marriage is supposed to be forever. You shouldn't be thinking "well, I'll try it and see how it goes". You should not be thinking about divorce at the moment you are being proposed to. If you react with shock and confusion, then I would venture to say that you are not ready to consider marriage. Don't say yes just to make him happy. Neither of you will be in the end. This is a HUGE, life-changing decision.