I know it is difficult, but don't fall for the whole "missing out" syndrome where you always worry about missing out on things. That only leads to anxiety and actually missing out on things because you feel that not doing things is a waste of time.
If you don't feel fulfilled, then find new hobbies that you can do at least 3-5 times per week. Most cities/towns have community-run programs for stuff. Even taking something totally random like a gardening, cooking, or photography class can be a fun way to kill time and meet people.
In the end, all our graves eventually go unattended and the mark we have left will mean nothing. So fill the time you have with whatever makes you the least sad, and do it until you die. Life is unfortunately a series of challenges and losses, and your twenties are hopefully a time where you can not experience much of that.
EDIT - Also, what someone said about routines is dead on. One of the reasons why time seems to go faster as you age is that you are doing mostly the same things over and over, and so your brain records less and less of it as memory. When you are 8 years old and have summer break, those few months are a significant chunk of your life, so it seems like the summer went on forever. when you are 75 and you do the same thing every day, then months can go by in a flash. This is because we only experience/remember time passing by the frequency of experiences and memories.
So, do new things as much as possible, especially outside of your comfort zone. This will make you feel like you are living longer and time will seem to move more slowly. It sounds weird, but it's true.
