I think what you’re thinking of isnt confidence, its more of motivation, but those might be the same thing I wouldnt know lol 
anyhow, when it comes to making games you CANNOT rely on motivation to help you finish your game, you need discipline :D
Ahem,
Motivation sets you in motion; discipline keeps you going.
~ Some Quote I found on Google.
point is, you’re going to have to learn to finish the project with out motivation, here are some things that have worked for me when dealing with this.
For starters lots of planning, I find that the moment I dont know what to do is the moment I stop working on the game and give up. So planning out as much as possible helps a load! A long with that when you do get stuck, just take a break and think about what exactly you need to do.
Having your project planned out also helps when in the case you are really tired of working on a certain part of the project, and just need a break from it, with proper planning, you can see other things that the game needs, so that way you have something to work on in the mean time.
Goals are also a big thing that can help a lot. Setting little goals for yourself can help you keep that motivation to help you reach your end goal :]
Building a community, like @Gobironlang said, before the release of your game also is a good way to ensure you finish it. First off, you have people who will play it which will help your game have a bigger chance to succeed. You might also feel responsible to finish the project cause you dont want to let your community down, which can overall make it easier to finish the project, at least in my opinion.
If you’re scared of your game being a ripoff when its done, somethings to help with that could be finishing your game as soon as possible, this gives less time for others to come up with a similar idea. Also if your game is just like, so much better than the game people might see as a ripoff I wouldnt worry about it too much lol.
Ok thats all I have to say, best of luck!