I have the same exact problem you can even see my post on the forum. What I do is I've realized that the increased heart rate does absolutely nothing, it can't kill you all it does is make you uncomfortable so I just keep telling myself it's nothing and that works. I also realize that weed can't kill you so whenever I'm feeling really bad I just tell myself it can't kill me and I just lay down somewhere, even if it's on the ground it's better to lay down on the ground then freak out.

Also don't take too many hits. Take like 1 or 2 hits and then save the rest for later, if you feel you're not high then take a few more, but if you start to feel panicky or increased heart rate then stop.

I considered quitting but I just said fuck that, getting high is way too good of a feeling to give up.