Marijuana can cause racing thoughts and can cause unwanted thoughts to appear as well. Everyone will say that different people have different reactions and this is true. So for your reaction I would suggest taking a LONG break. I too had a major freak out on weed last summer and pretty much didn't smoke at all until May of this year. I smoked some during the year at random intervels, but it wasn't much. That break really helped me to get back into smoking. I would also suggest that you smoke manily Indica strains if you can. They produce more of a body high and I believe is used to treat PTSD.

Another suggestion I will make is that you try meditation and maybe yoga. It can really make a difference if you are experiencing anxiety.