Try not to worry; priests are very experienced and deal with helping returning penitents make a good confession all the time. Also, please remember that you do not have to confess every sin you committed in those fifteen years. You only have to confess mortal sins. Mortal sins are those sins that are grave matter done with full knowledge and full and free consent of the will. A good examen should make note for you of what actions are considered grave matter. After you confess your mortal sins, if you wish, you might confess those venial sins you know to be persistent problem areas in your life, but you do not need to try to remember all of your venial sins. After your confession, if you remember a mortal sin you forgot to confess, do not worry, it was forgiven. Just mention it the next time you go to confession.
Two suggestions: First, if it would help you feel less nervous and less rushed, you can call the parish to make an appointment with a priest, rather than join the confessional line during normal confession hours. (Or you may prefer the anonymity of the confessional line. Whichever works best for you is perfectly fine.) Second, let the priest know you need his help in making a good confession. To request his help, you might say, “Father, it has been fifteen years since my last confession. Please help me make a good confession.” God bless.