At 5'8, a 54 is the right size for you. Since you're new to riding the bike is going to feel like a torture rack.... there's just no way around it. Assuming that you didn't cut the steerer tube, do you have all the spacers under the stem and have the stem at a positive angle?

If you do, and it's still uncomfortable, you just have to keep riding more until your body gets use to it. Road bikes are made to go fast, not made to be comfortable.