Sometimes life is the purpose. You are a unique individual, with unique qualities, and you are alive. That's special in itself.
And then are all the people you interact with. Whether it's family, friends, or just people you meet day to day. There can be meaning and purpose in them, especially if you can lighten their life, even just a little. A smile and attention to the person serving you in a shop can be special to them and be a reason to feel good about yourself.
To take it a step further, you might find meaning in a hoby, or in work, or in volunteering for something you feel strongly about.
I don't want to sound dismissive of your pain, or suggest it's easy to do any of this when you are low. I hope you can find the right thing for you, that makes you feel fulfilled.
Don't forget, if you need someone to talk to about how you feel, the contact numbers on this wiki are there for you.