I noticed when I am scared I tense up too and certainly get tired more quickly. Have to remind myself to pace, and loosen up the grip, arms, etc. I will unclip a foot if I think I need to but but that disrupts my connection with the bike. Going slowly is occasionally an issue for me. The other day I was climbing a steep road 2mph where I could hardly keep the front end down. I hit a little rock and ended up on the ground with the bike on top of me, still clipped in on one foot. Nothing bruised but my ego.