try to focus on the circle and focus on the rhythm, keeping a confidence also plays a really important role in keeping you calm.
I used to have that problem, but i always convince myself that i can fc easily and i do... not always on first try but neither after 100 tries, usually takes a few tries and im good (depending how hard the beatmap is), i always keep the same mood as i start the song with confidence and calmness, unless i am trying to fc a map that has a combo higher than 1000, then it gets tougher to remain calm, but it all comes down to confidence, experience and calmness
try also to take big breath with mouth and exhale with nose when you really are having hard anxiety.