I don't think that you are wrong, per se. Clearly, if Obama got depressed and conveyed a sense of doomed hopelessness, things would be worse. I guess I'm just too much of a wonk - if Obama gave a rambling, poorly received speech on how crappy our economic situation is that also ensured the stimulus package was about $400 billion larger than it actually was, well, I'd make that trade in a heartbeat.
Also, I'm not sure an attitude change will gain him Ben Nelson's vote in the Senate. I also don't think Obama's attitude has anything to do with his not adequately funding the infrastructure projects, R&D, etc. that you highlighted. That has to do with what he can get passed in Congress, how quickly those items can be put into place to deal with the recession, and how effective each piece can be overall.
Besides, we're already dealing with the issue of Obama's negative attitude: