ok, ive been playin guitar for about 4 years, and im pretty good. nowhere near as good as some, but i can hold my own.

Anyway, ive never been able to write my own lyrics or music/ when ever i try i cant think of anything else but a song i like and i end up giving up.

any tips?
Some people just aren't good at writing music. I'm not trying to discourage you, but it's true. My guitarist in my band is a good enough guitarist, but he cannot write music. I usually end up writing all the instrumentals. As for lyrics, my drummer usually ends up writing them even though I'm the singer.
it depends on what kind of "good" you are. If you are up on your theory then just pick a key play a chord progession and then add in different kinds of chords (sus, power, just octives to provide harmoney) and combine chords with little snippits of melody. just improvise and play all over the fret board. eventualy something should stand out and that should serve as insparation to find a sound/key to base the rest of the music off of.
If its not fender than whatever]