Dear Marty,
I've got a lot to tell you in this letter. A lot to explain and apologize for. Above all I want you to know that I valued our friendship, I value it, it isn't over, and I pray you will get the help that you need. Psychology isn't my area, it is considered in many circles to be a pseudoscience, but I must admit that the mind impacts many things, and yours has, somewhat predictably, turned on you.
Time travel isn't natural, and I never considered the fact that your mind might buckle under the weight. Then again I never intended on you traveling in time in the first place. I meant to go, and only me, because I understand that the smallest of actions can have dire consequences when you are acting outside of your natural time line. I suppose I should have been much more vigilant against all contingencies. But I wasn't, and it resulted in many alternate time lines in which you have witnessed things that were, perhaps, never meant to be, and how does one deal with that? As a scientist I understand that the world isn't quite what it seems on the surface, it isn't the world you thought you inhabited as a child. But you were still essentially a child when we embarked on all of this, you were only 17, perhaps not even out of the concrete stage of thinking. I knew you had trouble thinking in the fourth dimension, and this is what it has lead to.
The fact that there may be different dimensions in space and time is a concept I have been contemplating and dealing with for well over three decades, and I realize now that such things may have never occurred to you, never made up any part of your mind map of the universe. That may be why you have broken down over seeing your father dead and then seemingly resurrected as we travelled between alternate time lines. The changes in your parents' and sibings' personalities must have been just as jarring, the minute changes in the present brought about by things you did in the past to alter the original time line that you accepted as "reality" has negatively effected your mind.
I'm not sure how to restore you to your previous level of functioning. I suppose I'll just try to let you know some of the things that I know, and maybe in some small way it will help. This is a mysterious place, this universe that is just one of a billion times a billion universes. There are particles all around you that are so tiny we may never be able to perceive them, but they effect us, and everything else. What we know of these things, the scientific principles that seem to make sense in certain situations but that break down and fly apart in others, this is just the very beginning of the knowledge that may take us to unimaginable places. Remember when you were very little and learning the first letter of the 26 letter alphabet, A? That single A would lead to all the other letters in their various cases, and then to reading single words, then words strung together as sentences, then paragraphs, then books. From that single A you would be able to read the most startling poems and novels ever written.
I suppose what I'm trying to say is this. It is hard to believe, the things we've seen, but it is just another part of the unbelievable life we live, that we have achieved, that we are a part of. I'm sorry that my time machine has made you question what is, what is real, what can occur. Perhaps with time you will heal, and accept these things as just a part of the universe that none of us truly understands, not you, not me, not Albert Einstein or Richard Feynman, not any of us. Come back to me, Marty, because I miss your youth and your spark of life and your enthusiasm. Don't let the wonders and the horrors I have exposed you to take any of that away.
Yours,
Dr. Emmet Brown