I have a very, very old friend, we're very close. However in the past 18 months I've had a DC and moved away. I know she finds this hard as we used to see each other all the time, go on nights out, stay over each others houses when our DPs were working night shifts etc. Because of DP's shifts it's been really hard for me to get away and see her very often, we don't have other childcare very near. So now we talk a lot more on the phone than we ever did, but even that is tricky as she goes to bed very early because of her work, generally at about the time I'm just done with bedtime and had some dinner.
She's always been someone who's tended towards being negative. When we met in person I didn't really find this an issue, because we'd see each other for a whole night and she'd relax after a while and seem more positive about things, and I felt as though I had more time to talk through her problems if she wanted to (though sometimes I think she preferred just unloading rather than looking for help). However, now we're catching up mostly on the phone it just feels like 45 minutes of solid negativity, how everything in her life is difficult and hard and wrong. I'm not saying it's not, no one can judge that, but it makes conversations very difficult - I start to not want to call her, or feel I have to psyche myself up 
I know this is incredibly selfish and makes me a terrible friend. So, please, slap me, berate me, whatever, but tell me how I can manage this? I love my friend but it feels like it is becoming hard work.