I'm in my last year of undergrad so I feel that by now I should be absolutely sure about my career choice...Any advice?
First of all, this is completely false. You do NOT have to know what you want to do with your life after undergrad, relax. Even if you did know, some of the most successful people I know changed their minds (2/4 of the PIs on my floor at a top ten med school where I am a tech started off in medical school and decided to do research full time much later).
Few people want to go to medical school per se, most of us want to become doctors and medical school is a step along the way. Your job is to figure out what end goal you want (in whatever career) and then make it happen. 2 years is doable. Even if you don't want to be studying, you can do it for 2 years when you have a light at the end of the tunnel, a goal that you can watch yourself nearing. To that end, you can search for schools that break up the first two years with a lot of clinical exposure so that you have an easier time keeping your goal in mind.
I don't think you have talked to enough doctors. Don't talk to residents or med students - they are tired and overworked. Talk to someone who has the career you might want. Just like you got over the stress of applying to college and are happy you went (I hope), you will likely get over the stress of medical school when you get to practice medicine. In fact, I've seen some recent posts by medical students who are already having a hard time thinking back to the incredible ***t show that was their application year.
All that said, yes, medicine is hard. In all sorts of ways - surgeons have incredible physical demands, dealing with acutely ill patients takes a mental toll on physicians, certain fields require incredible amounts of training etc. It isn't for everyone and you need to figure out whether it is for you.
Take some time off. Get a job after graduating, get your mind off doing schoolwork all the time. By the end of my senior year I felt pretty burned out and didn't want to look at another text book. Now, a while out of school, I am eager to go to class and learn new things and even to do homework. I look back on my once-awful-seeming labs with fondness and miss the camaraderie of studying with my friends for a big test late into the night. Maybe a bit of time will get you excited too?