You have trouble forgiving yourself because you are INNOCENT.
The truth is, you do not need forgiveness. Forgiveness can only be given (is only necessary) when someone is GUILTY.
You did not force anyone to do what they did. THEY committed evil/wrong/bad.
Bad things happen all the time. Good things happen also, but you are not responsible for everything whether good or bad. The people who CAUSE/perform the deeds are responsible for them.
You did not rape the other girl either. The people who did that (the guys)...did that, committed that, and are responsible for making the rapes happen. If you feel responsible for what happened to you, then you would have to also accept responsibility for what happened to the other girl, but you caused no harm to the other girl (you know that you didn't cause her harm, so you should also not inflict responsibility for what happened to you on YOURSELF).
In justice (right and wrong), there is only
(you are not ready to be a criminal court judge until you get this straight...BAD/wrong is wrong).
Right is right and wrong is wrong. You were the VICTIM, not the CRIMINAL. You were RAPED (but there really is nothing LEGAL that can be done now because it was another country and a different jurisdiction, but it was also a long, long, long time ago, a legal proceeding would be simply unrealistic). Anyway, don't trouble yourself or regret the legal aspects, don't concern yourself about that at all, the important thing nowadays is that you are happy, married and a healthy mother (yes, you are important and deserving of happiness). Your happiness is the important thing and you should not let the past haunt you.
Let the past be the past, live in the PRESENT.
I have a past of 51 years of suicidal depression, but I am happy now because I ignore the fact that there is a past, I live in the present. I don't care that there is a past, I make believe that there is no past in my life (I didn't say it makes sense). You are in a different situation, you don't have 51 yrs. to forget/deny/ignore, you only have to release yourself from feeling bad about ONE situation that you were never really responsible for.
We can also declare ourselves guilty when a car runs us over or something falls on us or when someone robs us, but we were only at the LOCATION where it happened, we were NOT the CAUSE. Let the GUILTY be guilty, do not exonerrate them and accept/steal their responsibility for what THEY did. You have falsely accused the innocent (you are the innocent) and your conscience has always been mad at you for that.
RELEASE THE PRISONER, she has been falsely accused.
When something happens to you, YOU are a VICTIM and you deserve compassion/SORROW. MOURN/cry for yourself. It is not only OTHER people who deserve sorrow/compassion, you deserve it also. You are denying yourself compassion/sorrow. That is WRONG/unjust, STOP IT!, your conscience feels hurt by your refusal to acknowledge your own humanity. You would be compassionate and feel sorrow for other people, but you persecute yourself (not very nice of you). Treat yourself better.
There is nothing to forgive (unless you want to forgive yourself for falsely accusing yourself)...it might be sufficient to just be merciful and...
RELEASE THE PRISONER.