Didn’t read all the posts, so I’m sorry if this has been said, but I would suggest:
1. Try mindfulness meditation or another form of meditation, ideally in a group setting. Try at least 3 sessions before deciding whether you find it helpful. I’ve found meditation practice has greatly improved my ability to manage my thoughts, feelings, and attention, and repeatedly helps me deal with periods of depression, stress/anxiety, or apathy.
2. Service: I know this has been mentioned but must emphasize it. Transcending yourself and giving to a greater cause is probably one of the most powerful sources of meaning and fulfillment. Volunteer for any cause that you might be interested in: helping organize local races, coaching kids, restoring the environment, help at a hospital or charity,…
In both of the above, a byproduct of the activity is the opportunity to connect with people searching for deeper meaning/purpose. You will meet people from various backgrounds, which might bring you a new perspective on your own life, while also finding commonalities with them, at the very least in your spiritual or humanitarian pursuit.
3. Reflect on purpose/values: Actually take time to stop and sit and think about what you want to achieve in this life, how you want to help better this world, and what is important to you. And use this to decide what activities contribute to building your purpose or honouring your values. Repeat this reflection periodically and update accordingly. You need an overarching framework to guide you, otherwise you will indeed feel like you’re going through the motions without any sense of fulfillment.
4. Talk to people closest to you about how you’re feeling and consult a professional if needed (psychologist or psychiatrist).
5. If the opportunity arises to do so in a safe and controlled manner with trusted company, try an appropriate dose of psychedelic (e.g., psilocybin, LSD). Growing research supports their therapeutic efficacy for treatment or management of depression, often from a single session. Many who take them even for just a single session describe the experience as among the most powerful of their lives.