You know what, your advice is simple, but 100% spot on. Once I relaxed my jab became a lot sharper, and my 1-2 combos came a lot quicker. I just my hands dropping a little bit once I got tired, but I pretty much tucked the chin in all the way through as the guy holding the pads would throw back every now and then.
It was because when you make a conscious effort to do so, I think your body automatically tenses. I found not thinking about it and trying to be as natural as possible helped, and things flowed better
I've been training for almost three years now and this is still the most common injury I get. I think it's mainly to do with not holding a tight enough fist - something that is easy to do at the start of a round, but towards the end when even remaining on your feet is a struggle, it's easy to relax your grip and then get it caught. Well done though!