Is there a nice way to tell your mother that she’s pushing you away?

She’s a very negative person. All she really ever does is complain. If I say something positive happened to me, she’ll act happy for a moment, and then says something negative about herself. She’s always been this way but it’s getting worse. She’s upset that I’m moving out of state. I hate to say it, but I’m mostly glad to be moving because I won’t be exposed to her negativity on a daily basis. I try to bite my tongue but sometimes I want to snap at her.