Does the Catholic church believe that if you commit suicide you go to hell?
Suicide is a very difficult topic, especially for those who have suffered losing a friend or family member. Suicide can be a sudden act committed out of great despair, or the goal of a long planned trip to a clinic abroad where it is an escape from sufferings perceived to be unendurable in the future. Either way, when we are discussing a suicide we are never speaking in the abstract, but about particular people in very different circumstances.
Speaking of mortal sin and going to hell is speaking in the abstract. If a person freely (without acting through compulsion, illness, addiction etc) choses grave matter (an act which contradicts our human nature so deeply as to reject what God intends for us), and he or she also knows and understands what they are doing, then they commit a mortal sin, which needs the miracle of forgiveness to produce conversion of heart and a new beginning.
Taking any human life, including your own, is indeed a grave act, as contradicting the creative love of God which brought us into existence, and if it were to be done with full knowledge and freedom would constitute a refusal of that eternal love shown to us in Christ.
But when we speak in the particular, we cannot know what is really happening in any human life, and certainly cannot declare any individual to be in hell. We should always trust in the mercy of God who sees all things, and whose mercy is truly just, and who justice is truly merciful