Does your manager give you specific feedback that you can act on, or is it more general exhortations to "just get those times down"? Does he/she listen in to any of your sales calls, and pick out specific things you handled well, as well as specific things that aren't helping you hit call times/sales targets?
Coaching to improve needs to be about specific, timely, feedback that you can understand and act on. When I have coached people, we never aim to tackle everything - each week there's a specific task to act on.
That might be something like this (made up example): "when you talk about x part of the product, you sound doubtful and customers don't believe you even though we both know that your product knowledge is good. In the calls we listed to, you repeatedly said 'uh, I guess it's...' or 'well I think so'. Focus on eliminating those two phrases this week, and we'll review again next week. If you catch yourself using them, sit up straight, deep breath, and use the alternatives we rehearsed together."
Example is made up bollocks of course, but it is specific, gives clear actions to take, has worked through examples of good practice (not just "what you're doing now is wrong, do it better" handwaviness), and gives clear timescales.
Coaching is actually fucking hard to do well. It's one of those things that looks easy. But IME, even good practitioners (in various fields) often find it hard to analyse and identify what they're doing right, and what is wrong - often an inexperienced coach will be able to say X isn't right, but just won't have developed the observational and analysis skills to be able to pick out specifics - and without feedback on specifics, it's almost impossible to improve.