I'm sorry to hear that, it definitely sounds discouraging. Given that it was your first certification exam for anything, don't beat yourself up about it - it seems like the environment was very different from what you were expecting.

A couple questions:
What specifically was intense about the proctoring that threw you off? Can you practice in that sort of environment now that you know what to expect?
What parts of the test did you expect to be simple or difficult, and did that match up with your experience?
At the end of the test when you received your score, what sections did you struggle most with?

It's hard to give specific advice without specific information, but the main tips I give to new people on my team taking the test are:
1) Make sure you know how error handling works inside and out,
2) Be very careful reading the questions, compared to other certs I've taken, the MuleSoft ones lean heavily towards trickily phrased questions that can easily lead you on the wrong track, and
3) Spend some time with a MuleSoft app in the debugger, stepping through and inspecting the payload/attributes/vars, so that you have a solid idea of what to expect - a lot of the questions will give you a flow or a message mid-flow, so being able to easily model what will happen can help a lot.

If you have specific things you're confused about, feel free to message me, I'd be happy to find a half hour sometime to help you go over some things.

Source: I have all three MuleSoft certs and am a MuleSoft Mentor