Figure out how the negativity will bounce back. Your main character dies. Does someone else benefit? Was the main character partnering with someone, and the torch is handed to the partner who went on to do great things? Was the main character's plight written down in an article of a small newspaper, but then some person stumbles across the story and is impacted in a way that leads to great things?
Note that I'm not trying to say that the very negative thing (death) should be less negative. But don't let that be the end of your story, at least in your mind. Find some way so that when you think about the depressing concept, you can just remember that this is being responsible, as the death is an important part of a larger scheme, and then choose to focus on thoughts about the positive repercussions.
Then the pain may be less painful, so you can fully write things out.
I also have a few comments about:
I have whole scenes sitting in my head, dialogues, you name it.
Also, if you have great ideas bouncing around in your head, be sure to write them down. Even if it's just an outline/bullet points. You can flesh it out later. But once you've written them down, you can feel more free to think more about other things, instead of trying to remember details that you haven't yet written. It's quite liberating.
And then if life pulls you away from writing for a bit, but then you get back, you can think, "Oh, I had all these great ideas, but I forgot them, and now I have to go look at those notes I made to remember" instead of thinking "Oh, I had all those great ideas, but I forgot them, and now they are just simply lost forever".
Writing them down also gives you the flexibility to alter an earlier part of the story, and risk creating a scenario which causes your other good ideas to become non-viable, but knowing that you still have your bullet points that you can get back to. That means you have more flexibility when you're actually fleshing out your earlier ideas with words. Later, you can re-work in the details if you need to. But if you haven't written down those ideas, you may feel compelled to never get off track earlier in the story, because you don't want to miss out on some good ideas that you cherish. This ends up restricting your earlier writing, so it's not a good thing.
Remember, if you make a decision earlier in the book which ruins some of your later plans, you could still use those later plans by re-working them, or by simply using them in another story. Many successful multi-story authors have done that. So making choices that will complicate your ability to use some later ideas is an okay thing to do. Don't prevent yourself from making the most interesting choices.
The MC dies in the end.Spoiler Alert! Gosh! :)
I keep hoping that if I reach that part organically, I'll have the courage to write itYour current question shows that the opposite is the case. You're conflicted writing a happy passage because you know you're going to have to build a more negative future. Your negative emotions are caused by the negativity you're expecting between where you currently are and where you will end up. It's also a bit weird to write a story organically where the outcome is already fixed. That conflict might be adding to the issue.