How can one pay the Zakat for past years while having unpaid debts and interest-based loans, and will the Zakat be accepted in this case?

You must pay Zakat for the past years on the amounts you possessed, after deducting debts for each year. If you are certain about the amount for any year, then Zakat is obligatory on it. Otherwise, make your best effort to estimate the amount. Sheikh Ibn Uthaymeen mentioned that Zakat is a right of Allah and a right of the poor. Repentance removes Allah's right, but the right of the poor remains, so Zakat must be paid to them. To estimate Zakat, exert your utmost effort, for Allah does not burden a soul beyond its capacity.