
Real talk: motivation is not reliable enough for big projects. You need to use discipline to fill the gaps that motivation leaves behind.
A couple popular approaches are the pomodoro technique" or the "don't break the chain" method. You can even use both.
In any case, I think the key is responding to that feeling of "ugh, I don't want to work on this" with "...but I'm going to anyway, even if it's just for a little bit"