i understand you totally. sometimes where you want to play or obligated to play isnt the best opportunity to grow. its like a paradox. if you could just bring the house down with the one song u play, u'll definitely get to do more, but if ur just playing only one song, u'll never get good enough to do that.
what often happens is that we see what blocks us and it gets so much of our attention because we want it to change, that we often overlook the things around us that we can use to our advantage. You say you're playing behind other bass players. Are they better than u? If so, whats keeping you from looking to them for help in improving your playing. At my church I'm the only bass player and yeah I play the whole service, but as a beginner (2years on bass, but only 3 months actually playing) there are times when I just dont know what to do/play. If i had someone to look to/listen to and ask for help, I would be growing at a faster rate. Take advantage of that.
Sometimes you have to seek outside of your church to grow. But that doesnt always mean playing at another church. I have a long time friend, a professional musician who started teaching me the bass, who asked me to be a part of a concert he's having. i have to learn songs and practice, but i get to expand what i know and grow, and i dont have to leave my church to go to his church to play. Also Im auditioning for a gospel group as a backup bass player. now i might never actually get to play with them, but, touching on what i said earlier, their current bass player is a beast and i know i can learn so much just by being around him.
hope that helps u some in your situation