There are a number of people who have asked this question here, and the truth of the matter is simply that we cannot know what your family relationships are like, your reasons for needing to tell your Mom, or indeed enough about you as an individual, to be able to comment helpfully.
What I can suggest are a couple of modifications to your letter.
I do not believe in god. Not in the same sense that you do in any case.
Try to take the "you" out of anything 'negative'. In this line, you could replace it with the word "others". So it would say, "not in the sense that others do in any case". And "the story that you try to prove" becomes, "the story that people try to prove". Doing this, lessens the 'you versus me' position.
The other thing you desperately need to do is put in a bunch of paragraph breaks, because this block of text is really hard to read without them.
Lastly, and its just a thought, you might try just telling her first, and leaving the letter for her as a follow-up.
Anyway, good luck in your endeavors, and I wish you the best possible outcome.
Also, here is a link to another current TA forum discussion on the same topic, in case it helps.
Break it into readable paragraphs. One big block of text wil intimidate any reader.
Really good advice from previous responses.
I would personally try to edit this down to be more direct. Just as Elon Johnson said, you don't want to leave them unclear on your most important point, which is to communicate that you do not believe in God. While you do state this directly in the middle section, I feel like the overall tone is nevertheless equivocal and apologetic, rather than clear and informative.
Obviously, you mean to be sensitive to your parents' feelings and thankful to them for how you've been raised, which is great...but judging by my own parents, yours will probably latch onto this part: "This is where I am in my life. There are many things that I am uncertain about and many things that will change." To religious parents, this will scream "it's just a phase!" I simply want you to be able to communicate to them that you truly and honestly do not believe in God, based on everything you know about the world, and that having undergone this intellectual journey, you are unlikely to "grow out of it" barring some kind of miracle.
And do break it into paragraphs!
Best of luck!
I've been meaning to get to this, I don't know why I haven't yet and if it's too late then disregard the following!, but I'd make a few points and reiterate that everyone else so far has good suggestions.
My suggestions, mostly editorial:
Paragraphs are your friend. In case you weren't taught for whatever reason or other wise didn't know, when a topic changes, start a new paragraph.
The very first line would worry me if it were my child saying, "I am at a place in my life where nothing is certain." I think the first idea that would pop into my head is: does he even know if he wants to keep living?
The word "thing" is very ambiguous and you use it 13 times, which is a lot. "Thing" can quite literally mean anything and that can cause confusion for the reader. Specify whenever possible. This sentence, "So i looked at all things, and considered all things, as objectively as possible accounting for the natural bias that is a part of the human condition," makes me wonder what it was that you considered and looked at. By all things, did you mean everything that has existed ever, because that's what "all things" would imply? Or did you mean to say something more along the lines of, "I considered and looked closely at many of the world's major religions and histories as objectively as possible accounting for the natural bias that is a part of the human condition." It narrows it down and it's easier to tell what it is that you are talking about. Otherwise, I'm left wondering if you considered the mating habits of the Greater Short-horned Lizard in your search for understanding the world and finding your place in it. Now, not to mock you, but to give evidence to what I'm saying, I'm going to change some "things" to other words so you can better see what I'm saying.
"I have much to guide me in choosing these paths, i have my experiences and the sum of the microwaves I have [used] both through what I have been taught, what I have studied, and what I have seen with my own eyes. I have confirmed many delicious TV dinners that I had long held to be true but had never experienced personally, and I have changed my mind on many breakfast burritos I had been told were true. These were dinners that I took for granted when I was younger because they were meals that I would not experience for a long time, or were culinary travesties that were of no concern to me then."
This all serves to give me absolutely no idea what you are talking about until I get half way through when you say, "So in my attempts to create a more accurate understanding of the world, I have changed many long held beliefs, principle among those beliefs is my faith in god. I do not believe in god." The writer always knows what he or she is saying, but I as a reader and given only this letter, have zero idea that this is what someone is trying to tell me. Throw all that on top of a opener that has me worried about suicide and now I'm really concerned.
As Strega mentioned, toss out the "you." Using second person pronouns can put people on the defensive. It's a psychological mechanism that can get in the way of someone understanding what is being said. Use it when necessary as in the case of giving advice, criticism, commands or purposefully directing comments towards someone, but otherwise it should be avoided because it unintentionally directs comments at someone that they might find offensive or not relatable at all.
Over all, the second half is pretty good. It gives a brief overview of why without being disrespectful or giving an impression that she's dumb for still believing and ends on a personal and appreciative note. Clear up the first half and consider your wording so that the reader understands what is being said, make some paragraphs as necessary, and it'll be pretty excellent.
I have changed many long held beliefs, principle among those beliefs is my faith in god. I do not believe in god.
I know it's a nit... but it's principal when it's the most important or first item, and principle when it's a rule.
I also agree with pretty much everything that's been said up to now.
Despite the authoritarianism in the Catholic schools my dad sent his kids to, nuns again and again described faith as a gift. When I was able to shrug off the hellfire they'd promised, I started telling people no one had given me that gift.
I ignore the painstakingly crafted definitions I see on atheist websites.