Practice makes better, at the end of the day.
If you find yourself tensing up (I had this issue when I wasn't confident about my ability to play well on certain difficulties), trying to take it easy and think, but don't overthink, about how you should hit the keys next time to avoid messing up or locking up entirely. If you get too focused on buffering those thoughts in your head (if you're like me) you're more likely to do it again, and again- or find some other way to mess it up when you should've been cool, calm and relaxed.
It's my belief that the more comfortable you are with what you're playing, the more relaxed you'll naturally become over time. I took up the habit of playing harder streaming BPMs to 1) Scout out what I'd be heading towards next and 2) Build up stamina and speed. You don't need to 100% FC top score when you're trying to do these things- find something to improve on, improve on it, then more on. Repeat that with the same skill if you really want to.
Personally, most of my days I start off without being able to stream for jack shit. Takes a little while for my fingers to break through into the zone where I can play more consistently again.