My boyfriend and I have been dating for 9 months now. We are both 16. He's been talking to his ex this whole relationship but continues to tell me that she means nothing and always says she's gone out of his life. But I have a hard time trusting him when it comes to her. I love him so much and I know we are young but I can't stand living without him. I just want to be happy and I don't know what to do anymore.

She is still in his life no matter what he says, so decide whether you can go on like this and always be insecure and worried or whether you can do better than this. Think about it and do what your dignity and self respect demands. If you are that in love you take what he gives you and be happy and that's disgusting isn't it?

You say you don't trust him, but give not one clue why except that he talks to her.
Countless people are friends with exes, and it's harmless, because they REALLY don't have romantic feelings any more. Others do go over the line and stay involved.
If you don't believe what he tells you, then you are not in love in any healthy sense of the word love. You have three choices:
- find out what's wrong with you that can't trust, and do something about it,
- continue on in misery, mistrust eating at you, and fear that he for some reason still wants her (why? Makes no sense - he's with you),
- or you break up.

Actually continuing on will probably lead to him breaking up with you, if your mistrust and neediness for exclusive attention is obvious.
You will learn as you get older that couples who endure are those who have wide circles of friends both together and alone, and who enjoy the interest it maintains in each other.
Jealousy is a poison.