Also, I am usually very good under pressure. I've played football (soccer) for my whole life and am usually playing my best in the 90th minute in close games. I know BJJ is slightly different to football but the point is I've never really cracked under pressure until now. Even in my other BJJ fights, I've never panicked as I knew I had some control on the outcome of the fight by tapping. But in this instance, I didn't have that and he had regulation of my breathing which led me getting a little tunnel vision towards the end there. The point is, I think this was a one time incident where I lost my calm, I don't think tunnel vision is something I have to worry that much about in the future unless this type of choking is a common thing in BJJ.
It's not just about cracking under pressure as a whole like in a sport. You're going to be constantly putting yourself into new and uncomfortable positions. The first time you get a guy (or I guess gal) who massively outweighs you in north/south position and smothers your face with the girth of their chest or stomachs, it will suck. You really can not breath then. You'll have your face sat on. You'll have your gi pulled over your face which just make breathing harder. If it's just a one time thing for you, then great, but don't expect it to just be only this one time.