oof you're in a tough spot. Obviously I am biased, but I'd lose respect for someone if they were laughing uncontrollably at a disability; especially stuttering. If it were my gf and I told her I stuttered, I would undoubtedly question her response based on that other reaction. You have to tell her straight up, and I would tell her how you felt when she laughed at the video. Her reaction to that will let you know if the relationship is going to go anywhere.