You can ask to extend the due diligence period, but keep in mind that the seller doesn't need to agree to the extension... Also, it can take quite some time for changes to you credit score to occur. However, to request this, you would need to make an ammendment to the contract. Both parties would need to sign, agreeing to the new terms.
Best of luck
If the due diligence period is your only contingency period and it's about to be up, as the others have said, you can request an extension on the due diligence period, but the seller isn't obligated to accept it. My advice to you would be to be 100% honest with the seller and their agent about your situation so that they know why you're requesting it. It may even help to allow them to call your lender and hear it from "the horses mouth" that this is what you all are waiting on, as well as why they think it will happen so that the seller and the agent will put more faith in your request.
Good luck! If you decide you need an agent, ummmmhmmmmmm, we know a good one :)