They are all different careers -- you really need to do some soul searching and decide which route is best for you. I am an RN and do not particularly like being a bedside nurse because despite the progress of our profession, there is very little autonomy. I sometimes feel like a robot following orders. Yes, I am expected to think critically and the physicians I work with expect me to think about a situation before calling them; however, in the end, I am expected to follow their orders. However, being an RN gives you many different options and opens many doors.
A PA is very similar to an NP (both mid-level practitioners); however, the PA is taught using the medical model. In some states PA's are not allowed to independently practice whereas the NP is essentially able to.
You obviously know what an MD/DO is -- requires many years of committment of schooling and training. I am unsure if you have a bachelors degree already, but if not, you are looking at a long road ahead of you. I've met plenty of physicians who returned to school in their 30's, 40's and even 50's.
You also need to think of lifestyle. Are you accustomed to having a set number of days off every week and being home at a certain time of the day? Do you want an 8-5 job? Do you want to work three 12-hour shifts? Are you willing to work 80+ hours a week?
Also, if you pursue the MD/DO route, are you financially prepared to forego a salary for 4 years during medical school? Are you prepared to take out a lot of $$ in student loans?
Many many things to consider...
EDIT: I also wanted to mention that I really *wanted* to pursue medical school when I first started college and even during nursing school... however, as a practicing nurse, I am glad I chose to not pursue medical school. I have met very few happy physicians. Those physicians who are not office based spend at least 70+ hours a week in the hospital.