You seriously just need to relax man. I know it's hard for someone with anxiety to wrap their head around that simple fact, but it really is just that easy. You have to understand that you cannot OD on weed, and you have to place blind trust in your friends and dealers that your weed is not laced, because 99.9% of the time it isn't. Weed is too cheap of a drug for dealers to lace it. They'll lace blow or the more expensive shit because it's a different calibre of user that shoves that in their face. But this is just weed man.

You should be looking forward to that first toke every day. Let the reefer enter your lungs and slowly calm you down. Keep yourself busy when you smoke too, so you don't pay attention to every little nuance of your body. At least listen to some music so you don't have to listen to the voices in your head. Have you never slept on your hand and it's numb for like half an hour the next morning because of that? Happens all the time dude. Don't go running to the doc with every little issue.

And you can definitely smoke heroin my friend.