Acid burning sharp pain could be acid reflux, or you could have some thing wrong with your stomach? My Father-in-law suffered from acid reflux for years, could harldly eat anything and practically lived on rennies, eventually he went to a decent doctor and had an endoscopy thingy (swallowed a camera on a pipe) and they found some kind of infection that he's had for years, massive does of antibiotics and he can eat what he likes now (and he's put on about 15 lbs LOL) So it's worth getting checked out by many people in white coats if you haven't already.
The only other thing I can think that may be an issue is not enough fat if you are just eating meat and veggies(depends on the cuts and how it's cooked) that could be causing digestion difficulties. Do you have gall bladder issues? are you eating a lot at once, or eating lots of small meals? You may just have to keep changing things and see what works for you.
You know all those pictures of Adam and Eve where they have belly button? Think about it..................... take as long as you need........................