I know this is probably rather annoying advice to be given, but if you haven't already, you should talk with a therapist. You don't have to be sad to see a therapist.

Thoughts in your own head can erode very deep grooves in your mind that only direct more thoughts in the same direction. Letting someone look in and simply point out those tracks is theraputic on it's own even if it's easy to dismiss talk therapy as simply just talking.

Reality is just whatever your brain synthesizes from your senses, and that means you're never perceiving any sort of objective reality. We often think we know where those grooves are so we can avoid them, despite the fact that we're standing in deepest part of the proverbial canyon and simply pointing at scratches on the rockbed. We're predisposed to think in grooves we often think in, and our perception of reality resets to reflect that pattern.

A good therapist won't challenge "I am utterly alone in the world", because that's one of those scratches you already know "feels like a sad thought" when you think it. You have a lot of proof that it's true, but I'd bet you still recognize it as something hurtful to think about. Instead, they'd want to figure out what the last scratch was that deepened the floor of the canyon. Some thought/belief that you knew was hurtful and then was proven to be reality to you. Anyone can do that with you (friend, family), but a therapist requires no emotional reciprocation because you're paying them. It's worth it.