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If it has been done against your wish, you can take legal help. Are you intoxicated that time, or under any threat? Is there any injury or pain? Have you used condom?
If done with your consent, just driven by impulse, not need to get in guilt. Some time accident happens. Forget it like a nightmare or just an accident. In accordance with forgetting this try to involve in other pleasurable activity, be with a friend, take proper sleep and healthy diet, can take low dose anti-anxiety drug from your doctor.