In a way, this is good. Now you know what exactly is your problem.

If you've ever watched Arby n the Chief on youtube you'll know that Microsoft Sam has a very "charming" personality

Perhaps you should reevaluate your goals. If you did bad enough for them to say such a thing, maybe being a salesperspn is not the right job for your level of charisma. When creating your next character, be sure to put more points into that.

If this is truly what you want to do, then you should attend a few seminars, take classes, do something to learn the skills a bit more than you already have. After that, practice, practice, practice. You can never practice your sales pitch too much. Do it in front of a mirror, take notes, do it in front of friends or relatives and ask for input. And remember, an interview is just a sales pitch where you are selling yourself and your skills (Not like a street corner businesswoman, but it may feel like it sometimes).

Being a salesperson isn't about knowing what you speak off, it's about sounding like you do, and sounding good doing it.

Man, that must've been a real kick in the crowsnest.