Whenever I'm feeling overwhelmed, I just commit to doing 15 minutes of work. I have followed the FlyLady for quite some time, and I learned this from her. So I set a timer, and I just know that I can pretty much do anything for 15 minutes. More often than not, this gives me some momentum to keep going for a little while. But sometimes, that 15 minutes is all I can do. If you just take 15 minutes a day, for 5 days, that adds up to over an hour of work! It counts!
When we moved to our current home, I had a HUGE attic full of stuff that had to be moved, donated or otherwise dealt with. I had combined households with my husband and we had a lot of extra stuff from when we maintained separate households. I used the 15 minute approach to cleaning out the attic, and it was done before I really knew it. The first few times I set my timer, I sat up there with a garbage bag with my heart pounding, trying to figure out where to start. I would literally sit there and stare. The timer would go off, and I would make a bee line to get out of there, with an empty garbage bag and nothing done. I had committed to 15 minutes a day, and it took me 3 or 4 days to decide that I was just going to start filling up the garbage bag with things that needed to be donated. That was all the momentum I needed and then I was able to do the whole project over the course of the next few weeks.