One feature of psychedelics is that they enable (or force) the mind to drill down into very specific and granular thoughts and feelings, to assess a given thought or feeling from an almost infinite amount of perspectives.

You can't consciously choose which of your thoughts or feelings get interrogated in that way and it's very influenced by your state of mind and the environment you're in (set and setting, as Timothy Leary called it).

Therefore if your mind decides to focus on something happy, you'll experience indescribable bliss but if your mind goes somewhere negative you'll have an equal intensity of horror.

Often it's possible to work through the negative feelings but it requires submission. You have to accept what's happening to you and not shy away from it. That can result in long lasting positive effects because you'll have thoroughly worked through something that's been bothering you. But if you fight it, or you're otherwise not able to "solve" the bad thoughts it can have long-lasting negative effects because you've "proved" to yourself that it's an intractable problem.