It's completely normal to feel what you described; I've had it too. When you reach a place where life feels good, it's natural to want to experience as much of the wonder it has to offer and then lament how little time there may be to actually accomplish all that. And then I'd look back at the times I felt depressed and think "what the heck was I thinking? I should have started working on X or finally gone out and done that?" It's almost as though the up version of had little empathy for the depressed version cause when feeling up or normal there's a clarity that shows why the depressed state of mind was so... wrong. :-)

It's helpful to diligently finish tasks we start even though towards the middle or end of the journey they may lose the luster that compelled us to want to perform them in the first place. The reason this is important to me is because it affirms that when I get ideas to do something new, I do actually have the perseverance to get them done. That makes new prospects seem exciting again rather than pointless.