hey, man.. I've been playing for several years (not in a band though)--mostly learning songs and bass-lines written by other musicians, brushing up on technique, some theory here and there, etc... but I've hardly ever put time into writing my own stuff (songs, riffs, solos, etc), and while I've gotten to be a much better bass player over the years, like you, I still have trouble when it comes to writing or jamming out something off the top of my head.
A few things that have started to help me with the writing/creative-aspect of bass-playing:
-Starting simple... don't worry about trying to add in tons of different notes right away at the beginning. Just pick a few notes to work with, initially, and focus on developing a riff that is convincing rather than complex... I find it otherwise difficult to create a bassline that doesn't sound erratic and disorganized when I go nuts with too many notes at the beginning. Once you develop a solid introductory riff/part of your song, believe me, you will naturally have a much easier time trying to build on it's complexity, change things up or make it more interesting.
-Rhythm and feel... I can't start to develop any kind of riff or bassline without having some rhythmic "feel" or sense of the beat going on in my head. Unfortunately I don't have a drummer to practice with... but I've found that as long as I can establish a rhythm or groove in my head or be able to tap my foot to whatever I'm playing, a lot of the initial "writer's block" that I'm having goes away. Having this mental-rhythm is so important to me because it helps me stay organized while I'm trying to come up with a convincing hook/bassline, and furthermore, I've found that it helps me choose better: which notes to play at a certain time in the riff (especially when trying to work in notes that are outside of the song's key or "dissonant" most of the time), as well as changing the time-signature and working in fills or different note durations + dynamics.
These 2 things have really helped me shred some of my writer's block and start to think more independently & creatively. There are numerous other useful-general tips and pieces of advice that I could throw into this message (chromaticism and leading-tone concepts, tritone substitution and dissonance, etc. if you want to read a little further on your own), but I don't want to overcomplicate things in 1 post + I don't really feel like writing a novel, here (nor am I even qualified to haha). And keep in mind that everyone learns differently, so I can't guarantee that my tips will give you the same results, especially because I'm still learning myself. But I'm always happy to help out a fellow Jersey-native and hope that it might be of at least some value to you. Best of luck