Why do I always shy away from marriage?

Why do I always shy away from marriage? Why am I afraid of it? My first 'engagement' didn't work out because I literally 'ran away' from it… because I was afraid of marriage. 

Yesterday my present girlfriend mentioned wanting to come to my place and stay with me… in the sense that we should get married. And I was reminded of the previous incident where I had run off. 

I wanted to run off. It is not that I don't love her… I do… but marriage? I am afraid of it. I believe everything has its own place and time and enforcing something if the time isn't right will produce disastrous results. 

But when is the right time? Now isn't, for sure because I can feel it. How should I explain it to her? She thinks I am trying to go away from her because I don't love her.