How does one know if the reason for their prayers not being answered for years is a postponement until the the Day of Resurrection or due to a sin they are committing?

The response to supplication (Du'a) comes in one of three ways: either the supplication is answered promptly, or it is stored for the Hereafter, or an equivalent evil is averted from the supplicant. The supplicant must not rush the answer by abandoning the supplication, as this is one of the impediments to its acceptance. Nor can a person know whether their supplication is being stored for the Hereafter or delayed due to their sins, as this is from the Unseen (Al-Ghaib) that Allah alone possesses.