One of the most important issues related to constipation that frequently gets missed is thyroid function. Low thyroid function will cause constipation in certain individuals and to varying degrees. Make sure you get a detailed thyroid panel that includes the TSH, Free T3 and Free T4 levels. All three of theses tests are very important and absolutely necessary to understanding if thyroid issues are a contributing factor.
If you would like more information on thyroid issues and explanation of the tests I suggest, follow this link to view an article I wrote about this topic http://www.vitabase.com/blog/energy/sub-clinical-hypothyroidism.aspx
It sounds like you have tried all the natural and prescription remedies for constipation yet nothing seems to be working. You must continue to ask; “why is your body not responding and what is at the root of the issue?”. Clearly, your body should be able to eliminate on its own and there is some sort of imbalance occurring that is preventing or inhibiting this. How long have you been relying on laxatives (natural and prescription) to alleviate your condition? Long term use of either can create a dependency and make if very difficult to return to a normal rhythm.
Certainly there are other factors that may be an issue for you but I have had several cases of chronic constipation respond very well when underlying thyroid issues were resolved. This is likely a very good place for you start.
You may also want to consider food allergies as you mentioned you react to many foods. Doing an elimination diet for 3-4 weeks will remove the most allergic foods from your diet which may be irritating and disrupting normal function of your GI system. Dairy products, wheat containing products, eggs and soy are among the top most reactive foods. There are many resources available online regarding the proper way to do an elimination diet.