I feel such great sorrow for you as you continue to try and make sense of such a traumatic event. My heart truly goes out to you (you the adult and you the 10 year old).
I do not seek to excuse the final action a person takes in their life, I can however try explaining it in some way. Those moments, when everything seems darkest, the thoughts that run through someone's head are a true reflection of the overwhelming darkness of depression. The battle with depression can be a deep, irrational and seemingly never ending internal battle. It is a battle involving both thought and chemistry within the brain.
It is a massive sense of responsibility that you have lived with for so long, involving the belief that you could have managed your dad's depression. As a mum, I can tell you that the greater responsibilities in life should be shouldered by adults, for good reason. To give an example of the impact of great responsibility in a young person's life: From the ages of about 4 to 18 my friend's daughter played mediator to her parents before and after they split up. The domestic violence faced in this young girl's life as well as the role she felt compelled to play sees her now dealing with depression, anxiety and a lost sense of self. The complexities involved in great responsibility play out in a child's mind is such complex ways. Still, you are dealing with the impact of your dad's depression, to this day (in one way or another).
If you are still left feeling a sense of guilt and resentment it leads me to wonder whether you ever received the most effective form of guidance. It is never too late to seek constructive guidance, to find a professional who may be able to help you manage in reaching moments of greater clarity. In reassessing the relationship you have with such an incredible tragedy, you may find the way to a greater relationship with yourself.
Whether we're forgiving our self or forgiving someone else, forgiving always comes down to moving forward through giving our self release from something (letting go). The way people often speak about forgiveness can leave us believing that forgiving is just something we're expected to do, in a single moment, but forgiveness is often a process to be worked through, just like grief is a process.
Take care of yourself fish84 and seek out the guidance you are longing for, that which leads you to greater self-love and personal forgiveness