There are ways to control some of the things you are talking about, and there may not be a 'cure' to some conditions of BO. The problem with some antibiotic treatment is that is may throw off balance the normal microecosystem of your gut and mouth. It could create an overgrowth of candidas, or of some of the bad bacteria that normally lives in these places but is kept in check by the good bacteria. If this were your cause of odor, then you are very lucky. All you would need to do is to clean out your intestines until you get a one-time diarrhea and then replace it with probiotics. You can get probiotics at any health food store and whole foods market like Wild Oats. You can also get it online as long as you don't let it sit outside in very hot places. First take double the dose recommended in the box for about 2 days, and then just one a day.
For your mouth odor, you can get a tongue scraper at your local drug store and scrape off any excess mucous off your tongue that your body produces when there is an infection. I always brush my tongue after brushing my teeth as well. There's a relatively new over-the-counter product that seems to somewhat help called Oraljel anti-bacterial bleeding gum rinse, that is very good at preventing and treating oral infections.
Try these 2 management protocols to see if you get some relief.
Yes, your diet is not good...I know you're a teenager, but do try to eat more fruits and vegetables and less red meat :) Ideally, don't eat red meat. The diet is the tricky part because some people get odor from sweets, others from foods rich in choline like fish, red meats, broccoli, coliflower, beans, and others don't do well with dairy. Not everyone is the same. It would require a trial and error type of test on your part.
I'm glad your doctor will be treating your gastritis. You must try to bring down your stress level with some quiet time every now and then to relax yourself. Stress produces excessive sweating which can increase odor. Just find peace in your life as much as possible. Peace being inside of you - that's where you have to find it and nurture it.