sin - How can one make repentance for things he can't remember?

Say an unmarried Baal Teshuvah at the age of 30 decided to make repentance. A lot of time has gone by and he doesn't remember who he cheated, lied to, stole from, spoke loshen hara about, and so on. In such a scenario, is there anything an individual can do to make repentance for such sins?

Does the principle of erasing of all sins upon marriage apply for such sins?