If there's no chance of apologizing and you feel bad/guilty, then you need to get all the forgiveness from yourself and none from the other person.
We all blow it from time to time. We're human.
Keep telling yourself that the act was unintentional, mistakes happen, you're a good person who made one mistake, everybody makes mistakes. You might talk to someone about it, someone you look up to like a parent, mentor, religious leader. It helps to have someone else telling you that you're okay. Believe me, few people will be harder on you than you are on yourself. (So I guess you can get a sort of forgiveness from a third party, or at least sympathy and absolution.)
Also, you might think of it this way. If the other person doesn't care about you, maybe you didn't hurt him/her as much as you think you did.