The Man in My life...is wonderful. BUT.....?

Im 20 almost 21. He is 33. When we are at home just us its so much easier to be loveable. I dont like pda coming from us, i think it makes people feel uncomfortable so i avoid it. He has proposed to me and i have repeatedly changed the date and upset him, my mother and his mother with my changes. I feel that i am not ready and i explained this to him but he gets depressed when i tell him i dont want to talk about it anymore. I have my goals. I want to get into wildlife biology and travel, but if i get married and have a family, i will not be able to do the work i would like. I think i have worn out my best friend asking her questions repeatedly and i am afraid to ask my mother about it. I feel completely confused and terrified at the idea of losing my independence and personall will by getting married. Im also at the point where i consider him moe a friend than anything else and i fear for his heart. i care for him but i dont feel we will survive in the married world