I think it is simply a planning fallacy in action. More precisely: you are already in the middle of the old project, so you already see many problems, and the planning fallacy is weaker there. In the new project you haven't encountered any problem yet, so the planning fallacy is strong there. This is what gives you an illusion that the new project is much simpler. Maybe it is not on congnitive level, but on emotional level -- the old projects feels like "a lot of hard, boring work" and the new project feels like "exciting low-hanging fruit". Am I right?

At least for me this effect is rather strong, sometimes absurdly strong. I'll give you an example:

I was writing a book, hundred pages already finished, hundred more pages waiting to be written. My emotions gradually went from excitement, through "flow", to boredom, progressing to hate. So I stopped writing and started thinking: perhaps I should give up on this book and do something else instead. But what? What are my long-term goals and what are my dreams? So I imagined some possibilites, weighted their emotional appeal, made some connections like "this would help me do that" or "this and this could be made at the same time", and finally it became obvious to me that the coolest thing I could do would be writing a book, more specifically, exactly the kind of book I was writing right now. And the feeling was so strong, my emotions really wanted to stop writing the old boring book, and to start writing the new exciting exactly the same book; logically it did not make any sense, but my emotions felt that this time it would be perfect, because the imaginary book is always more shiny and easier to write than the real book. So... It's easy to see the mistake when your emotions are telling you that "X is much better than X". It's more difficult if your emotions, following the same algorithm, come to conclusion that "Y is much better than X". But the essence is that when you compare real with imaginary, the imaginary always wins. The new project seems better because it is imaginary yet.