I work in a similar place now and have in the past. I've taken the following approach as much as possible: 1) Use the opportunity to learn new technology and toolchains regardless of applicability. This can greatly increase your worth when looking for jobs in the future. 2) Try to find things outside of work to give your days meaning. Find a hobby or side project. I started designing board games. This lets you treat work much more as a 9-5 punch the clock task and lets you look forward to working on your real project later.

It is still entirely frustrating to feel like you spend your days digging holes and filling them back in but you have to fill your days with your own meaningful purpose.