so ive been jamming with my friend for a few days. the first day was awesome and we jammed out and we were both playing really good. the next day we jammed again and i ended up playing the same rhythm as last time. then i jammed again and i for some reason started playing the same rhythm. its only me and the drums cause we can't find a guitarist atm. but i play a little funky 16th note groove, then rest and let the drummer do a fill, and then i repeat. occasionally i'll switch it up with a simple chord progression change then go back to the original groove. i'll switch up the notes but when i try to change the rhythm my mind goes blank.

how can i change this
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try learning songs from genres you don't normally listen to. Learn some jazz, death metal, blues, country, anything and everything for bass, and try to incorporate new ideas into how your drummer plays. You don't need to stea any riffs or anything, but you can try some new ideas to get you out of a rut.
A trick I find is to practise specific rhythm patterms on your right hand. At the moment, I am favouring gallops on an 8th note, 2 16th notes and then 2 8th notes.

Mess around focusing on making a root note pattern interesting. Then add in melodic elements.
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Everyone has their own little bag of tricks and grows as you play more. It sounds like you are just reverting back to the same lick because it's what you're comfortable with and it sounds good. Just try and playing something completely different, ask the drummer to change up the groove and see what comes out of that. A changed drum groove can make you come up with completely new ideas. Force yourself out of your comfort zone.
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tell the drummer to start, and ask him to change up the tempo, then you try to fit it.
pther then that just get it stuck in your head that DONT PLAY THAT GROOVE!

I agree with this guy - Let the drummer lead and jam something that fits his groove.

You could even get a beat going in a drummachine/ dance Ejay/ powertab/ home keyboard/ etc. and practice Coming up with something on the spot.
Getting your drummer out of his pattern rut will help immensely, because you will have to adjust to his different rhythm patterns.

Also cycle through a song or jam / chord progression and make a conscious effort to mix it up each time you get to the beginning of the cycle. When I first starting playing with my current jazz guitarist, I found myself falling into very predictable comfort zones of patterns. He would break me out by yelling out different styles as we reached the turnarounds. It helped get me in that mindset and be a bit more creative and think beyond my usual paths of playing.