Your fiancee is 20 or so years older. She has lived a chunk of her life, probably accomplished some of her dreams, and is independent. She should be encouraging you to do that. Maybe she likes the idea of you being dependent on her. If you are not ready for marriage, you are just not ready. Be firm when you tell her no, you do not want to push up the date. Kudos to you for having goals and wanting to further your education.
I got married at 21, but I stilled lived the university life as much as my time and course load would allow. I never did anything that disrespected my husband or then-girlfriend, and I did cherish the life we had. No, I was not out until 4 in the morning and taking body shots from in between somebody's breasts, but my entire university experience was great. They were both slightly older than me, so they were in different chapters of their lives, but they understood that I wanted to experience those things, so they encouraged it.
I do think your fiancee is being a bit unfair. I guarantee if you ask her some of the things she did in her 20s, they will mirror some of the things you want to do and should be able to do as long as you are not disrespecting her, yourself, or doing something you would not want her to see/know about.. There is no rule that says you have to jam pack marriage, owning a home, having children, etc. or whatever in the first five years of adulthood.
I think your and your fiancee would do well to have a long engagement and adapt to the changes and additions in your lives before getting married.
I say go to school, live off-campus, if it is more cost effective, and grow into the person you want to ultimately be. Make some new friends, experience the life, join a fraternity, or whatever it is that you desire. Her support would be nice, but it is not mandatory. You can revel in her success and be just as happy for her whether you are sitting right next to her or away from her. Genuine happiness for someone's accomplishments does not mean you have to be right there or that you value what they have accomplished any less.