There is a solution to mental blocks, but unfortunately it's extremely extremely difficult:
Stop stressing about it.
The systems in your brain that deal with identifying and reacting to survival threats have, for whatever reason, decided that series is a threat. And the more you worry about the skill, the more it reinforces in your brain that it is indeed a threat.
So as much as you can, try to breathe deep and let the stress go. I know, easier said than done, but do the best you can.
From a coaching perspective, I'd do two things. First, I'd replace the series with something else. A handstand backhandspring is, developmentally, one of the best options for L7 beam series; however, there are other options that can still fit the requirements as a stopgap while you work through the challenge of getting comfortable with the series.
Second, I'd break it down and take a step back. Experiment to find how much it needs to be broken down to stop feeling scary. Can you do it on floor? On a low beam? What about doing the backhandspring on it's own without the handstand; are you able to do that? If you can isolate the exact step in the process where it starts to become scary, and break that step step down into drills and progressions and whatever else makes it feel less threatening, then you can gradually work your way back up.
Beam is not my specialty, so there are probably other coaches here who know more than I do about how you might break down this particular challenge. But the overall idea with any gymnastics challenge, be it physical or mental, is essentially the same: break it down into components see which components are still challenging, break those down into even smaller components, etc. Then build back up.
Good luck!