Please, first of all, accept our deepest condolences. Suicide is always tough for those who are left behind, so please know that we are keeping you and your family in prayer.
Catholics trust in the mercy of God. That is THE central element of our faith and Pope Francis has taken great strides to make sure that it is preached far and wide to the faithful. What we know about suicide from a psychological perspective today, is that it is often not a free choice, most often as a result of depression or other mental illness.
Then we add to our knowledge, what we consider sinful behavior. For something to be a sin, a person must choose to do this sin freely. They must be in control of their own actions. If they are not, then their actions are not sinful, albeit tragic in the case of suicide.
Suffice it to say that the church no longer teaches that those who commit suicide go to hell. Rather, we entrust them to God knowing that God can make all things new again. From the Catechism of the Catholic Church we read:
Grave psychological disturbances, anguish, or grave fear of hardship, suffering, or torture can diminish the responsibility of the one committing suicide.
We should not despair of the eternal salvation of persons who have taken their own lives. By ways known to him alone, God can provide the opportunity for salutary repentance. The Church prays for persons who have taken their own lives.
Since you asked for a prayer, any prayer for the dead would be fine to pray for your daughter. Here is one that I wrote myself recently for those who have committed suicide:
We entrust (name) to your undying mercy and love.
While in this life, they felt much pain and found life difficult.
May you enfold them now with your love where no pain can find them, but rather your love can heal them.
Make them Guardian Angels for those who struggle with life, for those who struggle to see you and the love that is around them.
Give them Lord, kind admittance to your Kingdom and bring comfort to their families.
We ask this, as we ask all things, through Christ our Lord. Amen.