I am 23 and been practicing yoga, kung fu, and zazen since I was 16.
Used to be very energetic, outgoing, and hopeful in life. Now it just seems I don’t want to do anything. Life just seems dull and boring.
I just don’t want to do anything more and want to just stop existing.
What should I do now?
Had you reached Nirvana you would not be asking on this forum if you had. It sounds like you are going through a nihilistic phase in your life, which interestingly enough I went through around your age too. It will pass. Simply change track on what you are doing currently and do something different. Shock your system, do something that you never would have in the past. Realize you have the power to play with your life and explore other possibilities.
Those are very different practices with very different intentions AND the labels in and of themselves don’t really tell us much. By that I mean just because something is branded as “Yoga” (for example) doesn’t mean there’s any yoga in it.
I’m not in a position to tell others what or how they are feeling. However generally a felling of hopelessness is a feeling of separation (rather than connection) and may be a form of depression, either temporary or ongoing. It should be addressed by a skilled teacher of yoga along with appropriate health care practitioners.