Instead of deleting your to-dos when you finish them, keep them around on a "Done!" wall or another place you can glance at to see how much you've accomplished. Seeing what you're capable of can give you a motivation boost and help you keep up the great work.
At some companies, completed projects and to-dos don't get erased when they're done — they're crossed out and left up on the whiteboard as a kind of "badge of accomplishment" or testimony to how hard the team has worked. The same could work for you. I worked at a company where this was a common practice, especially for long, drawn-out projects with hundreds or thousands of individual steps. 99U explains it like this:
Why not decorate your work space with completed action steps? While we tend to surround ourselves with art and imagery that serves to inspire us in our work, is it more inspiration that we need? Most creative professionals report that they are not short of ideas, but rather the discipline and organisation to make them happen. For this reason, consider surrounding yourself with testaments to taking action.
It can be really helpful to put an entire project plan or assignment list up on the wall, and scratch them off without removing the whole list. It's just as easy to do this with your own to-do list and a little space on a cubicle or office wall.