Well, you have learned one thing. Buying and expensive, lightweight bike does not instantly turn you into a fast road rider. No matter what, it should be more pleasurable than riding an entry level bike.

It takes time and effort to build up endurance and increase your speed. Google "local bike clubs" for your particular location and visit their websites. Look for one that separates their rides into categories by riding speed and then start with one of the more sedate groups. If you pick the fastest group ride you are going to be pushed beyond your ability and likely be left behind. Frustration like that take all the joy out of riding. Once you get better at it, move to the faster groups and enjoy riding.