I understand how to write one but when I try to make a bassline for it I always struggle, when im by myself. Im fine playing with a guitarist. Could this be because im not a guitar player myself when i paly the parts its just not interesting so it sounds bad?
Also I can write a decent bassline for a two chord progression but when I get into the 3s and above I cant seem to make it interesting, I just end up playing root note. I know scales and stuff but cant make anything except bassline that just loop over again after 2 bars.
I know this is fine and has a use but how do I write longer bassline that still work in the context of a song?
Have you tried chord inversions?
Are you familiar with Groove Me by King Floyd? What key is it in? What's the chord progression?
How about this?
Consider this series on writing for horns:
Try splitting the song up into parts for each instrument instead of having every instrument play the same thing. Think of each instrument as a different voice. Make space for each instrument to play. Think of rests, not just notes. Use intervals. Consider tempo changes, multiple time signatures, changes in the sub-beat pattern, and dynamics.
To go beyond two chords, you'll have to create transitions (turnarounds in blues speak).
BTW, compared to most pop music, you're setting a tall order for yourself. In much of pop music, the bass just follows the guitar. So, you might want to be careful about what pop music you listen to from now on. Much of it is very ... economical, frugal.
Consider listening to artists that write songs with more complexity than the average pop artist, like the Beatles or The High Strung:
You could write your songs for bass and make the guitarist come up with his own interesting stuff, MUHAHAHAHAHAHAHHAH.......
Most pop music is vocal music. So you might want to write a short vocal melody to start then switch to another pattern breifly to avoid too much repetition. Pay attention to what the vocals do in your favorite songs and how all the other instruments play along or against that.
You can use notation software to explore ideas. Most of them have midi instruments built in that you can use to play back your music. They do not always sound good. I guess if your song sounds good using midi instruments then you have something worth the band's time. Sound Cloud is one example. They have a free version.