Think of forgiveness as a process that can take time rather than a one-time decision. If you have a little sadness, this is normal. If, however, the sadness is deep and is interfering with your well-being, I suggest starting from the beginning and forgiving the friend again. Each time you practice forgiveness, some of the sadness may lessen. Again, please do not expect that forgiving will wipe away all feelings of sadness or even anger. If such symptoms are manageable for you, then you are advancing well in forgiving.
For additional information, see The Four Phases of Forgiveness.