You have received some great suggestions. parasites and heavy metals are real possibilities. Hair analysis tests are expensive, but they are an effective way to find out what is going on.
I would also like to suggest adding cultured vegetables (which you can make at home) to improve digestion. Making cultured vegetables increases the vitamin and mineral content of the vegetables hundreds of times and they should be used with every meal. Look up Donna Gates and the Body Ecology Diet. She has done extensive research and work helping people by re-establishing the proper good bacteria/bad bacteria balance. They also improve the immune system, so your body can fight off invaders more effectively. Unless you are eating probiotics (or preferably cultured vegetables) with every meal, you probably have an imbalance of good to bad bacteria and a weakened immune system.
You can find some videos on You Tube, where Donna Gates demonstrates step by step how to make cultured vegetables.
Good health is possible, but it does take determination to continue on, even when you are sick and tired of being sick and tired.