On maintaining creativity I have to say it's been decades since I have run a game that doesn't finish in more than a few sessions. Sometimes it's planned that way other times the game fizzles out.
I remember there was a game I was excited to run, but getting the group together became such a chore that by the time it finally worked on a schedule I just didn't care anymore, having had the idea in my head so long I had time to second guess myself made me not want to bother.
With shorter games it's an easier commitment for everyone, also easier to add or drop players. I have one or two longer games I would like to run, but they just aren't in the cards in the near future.
How I maintain the creativity is by prepping as little as possible. That's not to say no prep, but the minimal amount.
In the failed idea above I had over prepped, so much so I started to realize this was no longer a game I would enjoy playing in, making it worse to run.
Lighter prep keeps me on my toes more and improving with the group, prevents there from being a railroad because there aren't any tracks. I have always thoroughly loved Stu's advice to create problems not solutions. There is no reason to try and think out how the players will solve the issue, because you rarely think up the way they will do it.
On GM Confidence
Yeah, I think partially it is because when you are GMing you are hosting a party. A player can sit back flip through the rulebook, have a short conversation in character, or simple watch what is happening. The host is always checking on the guests making sure everyone has what they need. This constant need to be on stage at all times can prevent you from realizing how much fun people are having.
I ran a Jackercon game of KAMB. I bought the rule book the week of the game, said I can do this and ran a game five days later. I felt it was awful, but the players said they liked it and one player showed up at another session.
But sometimes you just second guess yourself into oblivion. I tend to move on to the next game and not let it bother me.