I don't know if I qualify for either . . .=)
Well, since no one has replied . . .
Bronze and silver are both available at the start of play, but . . . to open gold, you must beat the game on silver. To play platinum, you must beat the game on gold, and to play niobium, you must beat the game on platinum. Too bad you started in Bronze as you would be ready to play gold now. . . =). . . Altogether there are 5 levels of difficulty.
Because you discovered such a small amount of the world . . . I would say you focused on the main quest only (28 total main quests, 244 total side quests). Do all the side quests and you will discover more of the world and be at a higher level. (Each area has level caps, btw, staying in an area after enemies grey out is staying too long in that area.) I finished the Ancaria campaign at level 50 and the Underworld campaign at leve 63 discovering a little over 40% of the world. I did all of the quests except for a few that are broken.
I think SG always starts a new game when you change difficulty levels, could be wrong, others may have more info on that.
If you open the game, go to Ancaria campaign, click import, select your old character (only named character available if you have only played one character), change Bronze to Silver to continue on with that character in a new game, press Start Game, I believe he/she will be at the level you finished the Ancaria campaign, do the same for an Underworld character at different difficulty level. I would suggest you start again in the Ancaria campaign since the underworld campaign is so short.

Yes, you basically have to start over again when going up a difficulty level.
However, a large part of Sacred's charm lies in that you're not forced to do the main quest. Go out on your own, explore the world, and, like Stuff suggested, do the side-quests. There's a lot going on in Ancaria, ranging from the easy&silly (hint: look for a "Dumb oaf" in Porto Draco) to some neat chains (like the one given in one of the desert towns, don't remember which).
Also, you can set out to collect set and unique equipment (really, it can become almost an addiction...), at your ever-increasing level (since many different, leveled version of them exist).
You only need to do the main quest in order to advance to the next difficulty.
That is indeed so.

Thanks . . .=)