It took me 18 years to find the purpose of life. Fact just is, you are here. There isn't a real reason why you are here, but you are. You are because two humans mixed there chromosomes. As an atheist, I believe there isn't an entity that put you here. You just are.
So you can say the purpose of life is that there isn't a purpose. That can be pretty hard to accept. However, accepting it will result in you having no obligations to anything, or anyone. You can live life the way YOU want it. This statement can be interpreted in a negative sense, but negativity is resistance to something unchangeable.
Accepting the is'ness of life, without looking for a purpose, may grant you a life without negativity. Of course, it isn't simple to achieve, but acceptance will help you get closer to it.
I found out that a purpose or a problem or being proud, things like that, are bound to time. Without time, problems and negativity can't exist. If you don't agree, please give me an example. You will see that problems can't exist without time. Same as purpose, why need a purpose if you are here anyway?
Acceptance is the key
Existential depression and Insecurity
Try to keep smiling! :)