First off – all the answers above are very good answers. You are forgiven. There is no doubt of that. Whatever penance or reparation you were assigned, it’s done. Nothing more is needed.
So if you don’t have to do anything more about this, in God’s eyes, how are you going to convince yourself?
Well, obviously it will be good if you can do it by sheer force of will and faith. But you haven’t been able to do it yet.
Probably what you need is to do something constructive. If you can help people dealing with situations like your sin, that might do it. Or you could do something charitable, like helping with a food pantry, or a charity for single/unwed/widowed mothers, or St. Vincent de Paul Society, or whatever else your parish does that is charitable.
Enlist the help of the saints and of your dead friends and relatives in Heaven. They are cheering you on, and they will be happy to pray for you. So don’t forget to pray.
Saying “Jesus, I trust in You” whenever you’re tormented by feeling unforgiven might help.
Talk to your parish priest, or a wise friend or counselor who won’t pump you about the sin that was forgiven. Getting over guilt can take a while. You might need help and support from support groups, too. There are groups that deal with people who are getting over addictions, for instance, or there’s Rachel’s Vineyard for repentant people who had abortions. You can search for these kinds of things on the Internet without giving anything away, and your priest may know about such groups too.
I also strongly suggest that you get some holy water and use it. If the devil is tormenting you, or even if you are just a little vulnerable because of your sadness, you have every right to protect and bless yourself with holy water. It may also help you to have blessed salt, blessed palms, a blessed rosary or scapular or medal, or other blessed sacramentals, somewhere close at hand. (You probably have at least one of these things already. Just put it somewhere convenient for bad moments.)
The other option I would suggest, seeing as we’re still in summertime, is going on a short pilgrimage to some local shrine or outdoor Stations of the Cross or even a Catholic cemetery, or doing indoor Stations of the Cross in some parish you don’t normally visit, or even in your own parish. Going to one of the Doors of Mercy before the Year of Mercy ends – that might be a helpful pilgrimage, too. Even going in your car would help, but physically walking some distance (like in a cemetery) might also help. Walking barefoot is a good sign of penitence, too, if you don’t have anything wrong with your feet. You can pray for relief from your guilt, or for help knowing God’s will, or healing for your soul, or whatever you need to do. You can also sing hymns or do other pious activities. Not only will a pilgrimage give you time to pray and get help, but it may convince your body and brain that you can move along. (Don’t hurt yourself doing this; and make sure people know where you’re going, if you do it alone. Some people do pilgrimage bike trails, too.)
If you feel like you need to do a grander gesture, consult your priest/family etc. first, and be prudent about it. But hey, there are plenty of Catholics who’ve walked the Camino or gone to Our Lady of Guadalupe or even gone to the Holy Land, to help deal with guilt and shame. Can’t argue with the tried and true.
These are all suggestions. Don’t try to do all of them, but pick out some of them. (And definitely talk to your priest.) I don’t know whether you will get over this in a day, or whether it will take longer. But if you start to do something about it, at least it won’t be festering away inside you.