When you no longer "feel it" in your work or project, it's be time to move on.
There's a distinct quality of "feeling it" - an excitement, a sense of ease, and an increase in energy you get from doing something - that needs to be present for you to do your best work. Of course, there will be challenges and difficult times. But ultimately, you should always find a connection to the source of positivity, energy, and excitement.
Over the lifetime of a project, either you or the project may outgrow the other. The thing you used to "feel" no longer feels right. Instead of receiving energy from working on it, energy is drained. This is when change may be needed.
When you no longer "feel" something, it might be better to step aside. There may be someone else who will energize and carry out the work with greater quality than you can at that point.
However, this change requires courage. It's not easy to see your identity evolving and to make decisions independent of your past. We are deeply entangled in all we have done. But this courage is what will bring you to the next level of your evolution and allow you to find something you excel at even more.
To do great things, you often need to give up good things. Whether you're "feeling it" or not can be a great indicator for making that decision.