Nothing to be sorry over
Try some exercise, eat healthy, and try to avoid fastfood places. They can kill you, literally. I eat at McDonalds very ocasionally. Don't tire yourself too much after exercising - and eat some fruits, and nothing with alot of hydrogenated things (see the ingredients of all market products and boxes/wrappers).
You just started puberty a few months ago, and it'll take some time for puberty to end and for it to grow. Puberty ends in the late teens and early 20s, so you have plenty of time. Don't worry about it, it will DEFINATLY get bigger over time.