Hi Deniz!

I think the best advice can be provided once we get to learn your specific problems.

However, few points from my side in addition to the other answers that might help:

• Nervous: If you get nervous from seeing a lot of numbers - the best way to get rid of the fear is to just PRACTICE. Do a good number of intense math cases. That should give you the confidence.
• Flow 1: You need to first make sure that you quickly write your approach on the paper. No need to write full sentences - just do like a rough flowchart. Since you said your structuring skills are good - this should be doable.
• Flow 2: Write down all the data in the flowchart. If the data points correspond to actual boxes - write down against the boxes. If its an aside data point - then write in on a blank space. Once you use any data point - just put a mark on it. This way you will know what you have used and what you haven't.
• Flow 3: When you start doing the calculations - leverage the flowchart you have developed. Write down the intermediate results on the corresponding boxes so that you don't have to burden youself with remembering random numbers. This has 3 benefits: A) you won't have to remember anything except your approach; B) you will not miss using any number; C) you will be relaxed - knowing that you are proceeding as per plan.
• Speed: Accuracy is valued over speed. Assuming your speed is reasonably OK. If you are terribly slow then first get the math drills under your belt.
• Speak aloud: Check out this Q&A https://www.preplounge.com/en/consulting-forum/doing-calculations-in-silence-or-speak-out-loud-10992
• Rounding: Its eventually an art. You will need to practice to develop the right judgment. When you are practicing - do it both ways and see how much of a difference your rounding made to the final result. With enough practice in place - you will subconsciously develop a sense of what to do. Sometimes 3.9 can be obviously rounded to 4.0 - but sometimes make keeping the 3.9 as 3.9 might be the right way (either because its really important to keep it 3.9 - or because it will make your calculations simpler.)

(edited)