If that is the case, then you need to reduce your inventory. So that you only have sufficient stock there, for maybe one month of sales and not an awful lot more.
Reduce your inventory at FBA. It’s only 25p (+VAT) per item to return it to you.
I’m guessing from your name, that your in clothing and that if so, you probably have a lot sat there, that are doing not very much at all.
If that is the case and you want to offer Prime delivery, why not look at using Seller Fulfilled Prime instead? No storage fees and just as much visibility.
As to a mentor, unless you know someone local, or a willing to pay one of these internet “Amazon experts” (Definitely not recommended), then just ask your questions on here. There is a lot of people that will help you. Also, take a look at the seller university. Lot’s of information there that will help.
Amazon is quite a steep learning curve, but once you get over the hurdle, it’s relatively easy to keep things in check.