With respect to what constitutes destroying a soul:
Primarily it is referring to taking a life. There are other meanings, especially in the reverse (rescuing a soul), that refer to spiritual matters, but these are things having to do with one's spiritual growth and development. If you act as a mentor to someone who seems lost and wandering in life and assist them to become good, upstanding members of society, particularly if this person is Jewish and your guidance leads to their living a more religious and spiritual life (and assuming this is a positive and uplifting experience for them) and this carries forward to their family and children and grandchildren, then I've seen people apply this concept to that mentor (or those mentors) who helped facilitate this.
With respect to making amends:
One can only do what is in one's abilities. If you have driven someone away from living a positive and fulfilling life, Ḥas VeShalom, then you certainly ought to do your best to find the person and make amends. However, if you cannot find the person, I would advise you to speak with a religious leader about ways to repent without apologizing. Even if you can find this person, it is possible that the impact you had on his or her life was so detrimental (as implied by the question) that he or she may not forgive you. There are rules about seeking forgiveness on 3 separate occasions and, afterward, the sin reverting to the other party who won't forgive (RaMBa"M Hil. Teshuvah 2:9). However, I would caution you that seeking forgiveness from someone who was deeply hurt, and doesn't just have some unreasonable grudge for some slight years ago, is not likely to be met with appreciation, and that person is not likely to forgive at all, and this rule may not apply. Again, I would advise you to seek guidance from a religious leader whom you trust.
Although taking someone's life is considered something for which one cannot really make amends, and which Yom Kippur itself does not atone, if one is truly remorseful and repents for it to the best of one's ability, then, according to the RaMBa"M (ibid., 1:4) any suffering experienced one's life will provide provisional atonement, which will be granted at the end of one's life.
Finally, RaMBa"M advises (ibid., 2:11), that if someone sins against his fellow, and the fellow dies before the sinner can ask forgiveness, that the sinner go to the person's grave with 10 other people and confess his sin to them.
A free translation of the formula for the above request for forgiveness at a grave:
"I have sinned to HaShem, the G-d of Israel, and to this person named [so-and-so], in that I did [such-and-such]." At that point, if he owed the victim money, per the RaMBa"M, he should pay it to an heir; if there is no heir, so far as he knows, he should deposit it with a Beith Din and confess.