Mole Sounds

I wish to voluntarily register for VAT, even though I am a small business. I have imports from the USA and outside the EU that I have to pay VAT and duty on, these are growing. Normally I will of course build these costs into my RRP, which can make them expensive. I am confident that going forward I will be able to manage all this. However, what do I do about VAT that I have already paid on existing stock? If I add sales VAT to my current stock when sold, how can I offset this on the VAT paid on the purchase? I can adjust my selling prices to account for the VAT, hence making me still competitive price wise, but surely I am then paying back HMRC and have no means to recover this?