Theré's really no easy answer mate, but my advice is , don't get down on yourself, your 19, it's difficult to get any job where you feel really valued at that age, god knows I remember the crap pay, the crap treatment by bosses and other members of staff, simply because your young! You may be doing the same job better than say a 40 year old, and yet he gets more money, and all the respect, it's pants, but it's how life is sometimes! Stay with the course if you can, especially if you feel you are gaining something from it, if you are finding it difficult ask the course tutor for more help, that's what he's there for!
This situation won't last for ever I prommise, as you get older you will find out where your skills lie, and develope them, all of a sudden you will be a valued employee, and more importantly you will feel valued!
Okay the op, I'm sorry to hear you are ill, but that doesn't have to a completely negative experience either, okay you have to face the surgery, and that won't be pleasant, but it's going to fix whatever is wrong, you will have a little time, whilst you recover, to reflect on what you want to do with your life, once you are fully recovered! You never know you might meet a beautiful nurse!!