I think you've pretty much made that first most important step: realizing that you're being overly negative, and deciding to actually change that about yourself.
You may wish to to communicate this goal to the rest of your team and get them on board with changing that about yourselves, because otherwise you risk getting frustrated when you try to be Mr. Optimist, and everyone else shuts you right down. However, if you think they'll get offended if you take this approach then you simply have to commit to becoming the office cheerleader.
The next time a daunting project comes up try to take charge of the conversation and challenge people to think outside the box, and be creative rather than give up:
"That feature is too big"
You: So guys, how could we break that up into more manageable chunks? Could we slowly build it behind the scenes, create duplicates of some methods, and then eventually just go live? If our lives depended on it, how would we do this? Best idea gets a beer on me at lunch on Friday!
(alternatively: let's come up with a solution, and if we do, i'll bring in donuts tomorrow morning!, etc.)
"We don't have enough details to get started"
You: I'll get a meeting set up and get more information. In the mean time though, how do other applications/websites/etc. implement this sort of feature? What sort of things should I ask them about in the meeting? Let's come up with a list of possible features and functionality so that I know how to steer the conversation.
Again, get them thinking about a solution instead of simply giving up.
"This would be easier if we had "x""
You: Yea, it would be, but since we don't have it, how could we MacGyver some similar functionality? Who's gonna be the MacGyver of the hour?
Just try to inspire them and get a little bit of a challenge going.
You may have noticed I'm not exactly a behavioral psychologist. I would suggest Googling team building exercises and other things like that - I'm certain you'll find a lot of resources relating to this topic.