im not bad at writing songs but i alwayse have trouble with coming up with the guiter rythm

any suggestions?
Take yout time. Listen to music, and get inspiration from various genres. Spend time just noodling randomly on your instrument. Not many of my riffs came to life consciously.

Sometimes it's also good to know what you want. When I write lyrics, I often hear melody and rhythm stuff in my head. Then I consequently convert it to my guitar.

And another suggestion - find someone with a bit different approach to work with. I often compose with my brother, who is more glam oriented. When one of us drains completely, there's always another, who can "carry the torch" and finish the piece .