It sounds like you are struggling to convert word problems into equations - this is pretty essential both in the interview stage of consulting, and on the job as well. It's great that you have identified this as a development need.
I would suggest practicing lots of word problems (e.g. GMAT preparation materials should provide some examples, or the McKinsey PST) and getting comfortable with writing formulas from word problems. There are a few formulas that you could "learn" to help with common types. If you are struggling, have a look at the GMAT solutions - these are usually very clear in terms of method.
As with many quantitative skills, this one gets better with practice!
Best of luck,