Other than using a coach (and here I mean those that use a mentoring approach, not the “interview roleplay” style), the best thing I can suggest is to PRACTICE ALONE AND READ CASES.
Stop practicing with interviewers until you get this right. Read the case. Stop where the quantitative questions starts. And try to answer the question by yourself. Don't give up easily, i.e., if necessary try for 20 mins before you reach a conclusion or read the answer key.
Maybe equations is your problem here. But you can spend those 20 minutes trying to come up with equations.
Remember you are training your brain here. If you can't solve these on your own, it makes no sense to be practicing with someone else. It's a waste of your time (and theirs as well). You should only practice with someone else once you know what you are doing, otherwise you will be operating at the wrong difficulty level (with peers you train communication, time pressure, and the surprise element… not the basics).
And you have to try hard by yourself before reading the answer, otherwise you are not training your brain (it would be like trying to improve your soccer game by watching TV…).