Relative Pitch

I was just curious... recently I've been having alot of trouble with writing songs, I can write PARTS of songs no problem but putting them together and creating lyrics is another story. I've actually noticed too that I have trouble singing as well, I don't always stay in key (which would be very embarrassing if i was a singer, luckily i'm not). So one of the things I want to work on is getting better relative pitch, since I heard that getting perfect pitch isn't usaully something you can train and also a number of people on the forum have said that perfect pitch is more of a curse then a gift.

One of the things i'm going to do is everyday concentrate on one or two intervals, both melodic and harmonic, and just try to concentrate on how they sound. Are there any more exercises that I should try? Does anyone have any other ideas to improve my songwriting? because I'm pretty much at wits end, and if this doesn't work then i don't know what will.