I am a veteran of the current Iraq war. I am very sorry that you have had this experience. However, I would like to offer a few suggestions that might help. I know that it is difficult for a lot of veterans to obtain benefits from the government because my job is to help military and veterans. Although, keep in mind I don't know everything about your situation so I am only writing based on what I saw you post.
To start with I have a few questions for you. When you filed your claim with the VA did you file it with the assistance of a veteran service officer for example a VFW or American Legion Rep? If not, I would highly suggest talking to one soon about your experience. They may be able to offer suggestions and assistance with filing your claim it is what they do all day long and they are usually very knowledgeable about works and doesn't work in getting claims through the VA. Also, it seems that because this problem started after you left the military that it may be more difficult to prove service connection in your case because that is what the VA uses to grant disability ratings. However, in your case the government has acknowledged that you were exposed to sarin gas. Another option you should consider is seeking the assistance of an attorney. There are many attorneys that offer pro-bono assistance to vets in filing claims with the VA. I hope this helps. Good luck with your claim. I wouldn't give up in fighting for your benefits. I'd look at it as part of the process of serving in the military and being a vet. I know it sucks, but it's the world we live in.