First thing I would do would be to change her food. You're feeding her the equivalent of McDonalds. Check out www.dogfoodadvisor.com
for options. Many feed Taste of the Wild which is a decent quality and price kibble. Make sure when you switch, you do it gradually to reduce the chances of diarrhea. You can add a tablespoon of pumpkin (real pumpkin not pumpkin pie filling) to her feed to reduce the likelihood of diarrhea and firm her up some if she is loose.
Also three cups of food is too much anyway. Especially for a relatively inactive pup. My two year old 50lbs mutts only eat 2 cups a day and they are very active. My older girl at 70lbs eats 2.5 cups a day. I think you will find changing her to a higher quality food will improve her energy level. Follow the feeding recommendations on the bag but make sure you understand that the feeding guidelines are total amount of food for the day. At 5 mos twice a day feedings should be good so if the feeding guidelines are 2 cups per day feed her 1 cup twice a day. If she really is a low key, relatively inactive dog then err towards to lower side of the guidelines to keep her from getting fat.
Also following the recommendations made by BD. You definitely want to stop carrying her and start training her. You need to establish that you're in charge and she will follow your lead. She's young and should be easy to train/fall in line quickly.