You're not without purpose! You may not SEE your purpose yet but I promise you, I swear to you, you have a positive purpose in this life.
Please, please, please call a mental health outline and talk to somebody. It will help! We all need some help sometimes, it isn't unusual or bad to reach out, like you have here...but having a live person in real time, who is trained in the field and is familiar with issues that you are coping with will help you immeasurably. Again, you might not feel or see it immediately, but it's that contact itself that can make the difference.
Prayer will help, but sometimes people become so immersed in prayer that they neglect to take the action that is needed to pull themselves over the bumps...so mix up your prayer with ACTION. Even if it's small action. Walk to the store. Call a friend or relative (or the HOTLINE!) Catch a movie, hang out at the park, visit a homeless (or animal!) shelter...do something!
Just physically moving to a different spot (going from inside to outside) can help. Instead of mulling over the reasons not to live, just give yourself one thing to do, and do it. Put everything on the shelf, and do ONE THING.
Question...do you have a job? Are you a student? Why do you think you don't have a purpose?
There is a national collaborative hotline called 2*1*1...I suggest you contact the nearest one. It's a hotline with myriad resources...you tell them what you need help with, and they will hit their data base hard and transfer you to the person you need to talk to.
So if you don't want to talk to a mental health hotline, for example, and you're depressed because you need medical care, they can refer you to the appropriate person to help you with that. If you need child care, they can do that...it's a really wonderful collaboration!
http://www.211.org/
that's the link, type in your zip and a little pop-up comes up saying "call 2*1*1" lol...I typed in my zip just to check, in case there were some blind areas, but it came right up.
Good luck and God bless you!