There doesn't seem to be anything really difficult in life that I can't get through, and nothing particularly unhappy happens, but every day I feel sad for no reason, and I often even have thoughts of dying, thinking that there is nothing worth looking forward to in life.
I really didn't know what to do, and the whole thing was in a funk.
Tried running to ease my mood, but it didn't work very well; heard that taking vitamin B12 might help with depression, but it felt like more of a psychological comfort effect.
I'm desperate to find some way, even if it's just psychological, just to ease this pain.

Usually when nothing happens, my mood is like a pool of stagnant water, so low that I can't move; once I encounter something, I may have suicidal thoughts, and even think about the method and the details of the suicide note in detail, but fortunately I can still control myself not to actually act.
Sometimes I would also want to vent by self-harming, because attacking myself after being stimulated by the outside world would instead give me a temporary release of stress.
Hi, first of all, a warm hug to you! It's great to see you asking for help, it shows that you're still trying to find a way out in the midst of your pain. I want to give you credit for choosing to seek support at such a helpless time.
If a psychiatric hospital has diagnosed you with a state that requires medication, then taking your medication on time is an important step, so try to follow your doctor's advice. At the same time, some cognitive and emotional adjustments in conjunction with the medication will be more helpful.
Try to become aware of whether there are certain unmet needs hiding behind those thoughts of wanting to die? For example, is it a desire to escape a stressful job, or a desire to be more true to yourself by not pushing yourself to maintain an unhealthy relationship?
As you quiet your mind by taking deep breaths and slowly exploring the reasons behind these thoughts, you may be able to be more accepting and understanding of your state, and thus find healthier ways to respond to these needs.
As an example, you can intentionally recall moments growing up that made you proud, think about what you did at the time and how those around you responded; or find your hobbies and use them as a distraction when you are feeling low. You can also try to share painful feelings with friends and family, or record them in a journal. Thinking back to what you were doing in the past when you didn't have these emotions may give you some clues.
I'm Lily Little Ears from the Answer Museum, and remember, the world and I care about you.