A motion for summary judgment will be denied if the non-moving party is able to demonstrate a bona fide dispute as to the material facts. A moving party who can show a certain amount was owed, as well as an agreement stipulating to the applicable interest rate on the unpaid balance and any late fees, and through this method demonstrates how the figure claimed to be owed was legitimately derived, is in a strong position to demonstrate undisputed facts warranting summary judgment. However, if the nonmoving party comes forward with other evidence, showing, for example, that certain payments made were not credited, or a different interest rate applies, or some other material fact which creates a dispute which can only be resolved at trial, then the court will likely deny summary judgment. Conclusory statements in an affidavit, otherwise unsupported by material facts, such as a blanket averment that "I believe I owe much less," probably will not suffice.
Not legal advice as I don't practice law in California. It's just my two cents on the facts you describe in light of general principles of law. If you need legal advice, please consult a lawyer who holds California licensure. That's not me as I practice in Vermont ONLY.
I would suggest seeing a debtor's rights attorney immediately. Filing an opposition to a summary judgment on your own would be highly inadvisable. A motion for summary judgment deliberately requires that facts not be in issue (i.e. not contested by both parties]. A debtor's attorney will understand how to successfully defend such a motion. You do not want to defend this on your own.
Best of luck in obtaining a favorable resolution (with proper legal counsel).