Why is the enamel on my teeth chipping off so easily and eroding. I wear a biteguard and do everything I am suppose to do, do I just have weak teeth?

Imbalance. When teeth, jaws, and muscles are out of balance, teeth are subjected to excessive wear. Trauma, grinding, clenching, large old fillings, eating hard candy/ice, can all fracture the enamel. An occlusal orthotic to bring jaws/muscles back into balance is good 1st step. Occlusion can then be evaluated to see if ortho rx, restorations, or both are needed.
Are you grinding. You might have thinned out your enamel previously due to grinding previous to your bite guard use. Also make sure you don't have a traumatic bite that causes the edges of your teeth to break frequently.
So many reasons. Basic answer is that every individual has a combination of factors that contribute to his/her wear, attrition, fractures, etc. Your stress, your malocclusion (bad bite), the foods you eat -- like ice chewer, or peanut brittle/ etc., even lifting heavy weights will cause significant forces in your mouth. Talk to your doctor about getting a stable good bite, and then look into foods, guards, et.
Enamel chip. Your problem could be your bite. The wear might not be from you grinding. If your night guard shows very little wear, you are not grinding. Only 8 % of the population actually grind their teeth because their brain tells them to. A lot of tooth wear comes from an uneven bite or the front teeth are too tight in terms of position. Unfortunately, occlusion(bite) problem is not well understood .
Diet and acids. Your diet will have alot to do with this. Highly acidic drinks and foods will erode and weaken your enamel. Diet drinks, gatorade, energy drinks all. Are very acidic. Dry mouth caused by many medications will increase this. So will gerd. So look at what you are eating, how often you eat and drink acidic things. Talk to your dentist about mi paste or Fluoride to reduce this from happening.
Enamel chips because. Teeth, joints and muscles should work in harmony. Excessive tooth wear means this functional balance between your teeth, muscles and jaw (joints) is out of whack. Most probably you need to find someone that can help you find the best functional relationship of your jaw and muscles and be sure the bite supports this relationship. An orthotic is a diagnostic appliance to help find this position.