Praise be to Allaah.
Allaah says (interpretation of the meaning):
Those who eat riba (usury) will not stand (on the Day of Resurrection) except like the standing of a person beaten by Shaytaan, leading him to insanity. That is because they say: Trading is only like riba, whereas Allaah has permitted trading and forbidden riba. So whosoever receives an admonition from his Lord and stops eating riba shall not be punished for his past; his case is for Allaah (to judge); but whoever returns (to riba), such are the dwellers of the Fire they will abide therein.
Ibn Katheer, may Allaah have mercy on him, said concerning the phrase So whosoever receives an admonition from his Lord and stops eating riba shall not be punished for his past: This means: whoever hears that Allaah has forbidden riba should stop as soon as this ruling reaches him, but he can keep whatever he had received before, because Allaah says (interpretation of the meaning): Allaah has forgiven what is past [al-Maaidah 5:95]. Similarly, on the day of the Conquest of Makkah, the Prophet (peace and blessings of Allaah be upon him) said: All riba owing from the time of Jaahiliyyah is under these two feet of mine (when he abolished everything that was owed to the usurers except their capital), he did not command them to return the extra money they had received during the time of Jaahiliyyah. (The phrase) (Allaah) has forgiven what is past is akin to the phrase his case is for Allaah (to judge). Saeed ibn Jubayr and al-Saddi said: Shall not be punished for his past (means) for what he used to consume of riba before it was prohibited.
(Tafseer Ibn Katheer; the material in brackets has been added for the purpose of clarification).
On the basis of the above, you do not have to repay the interest which you received before you found out that it is haraam, but whatever you have received since then must be repaid if you still have it. If it has become mixed with your own money and you do not know the amount, then try to estimate what you think is most likely, and pay that amount.
We ask Allaah to accept our repentance. May Allaah bless our Prophet Muhammad.