2. FIND MOTIVATION: Get excited about why you started in the first place. Go over the plan from scratch with a co-founder, talk to users, visualize the end goal, use the product, or do whatever it takes to get back some of the initial excitement.
3. BUILD MOMENTUM: Be as self-indulgent as necessary at first. Don't worry about doing the highest priority tasks, just get going. Do whatever part of the project seems most fun/easy/interesting, however trivial. Do whatever it takes to start building back your speed. Once you're at speed, the hard stuff won't be so hard.
Now you're relaxed, motivated, and have momentum so it's full steam ahead!
Fall off the wagon for some reason? Don't waste time feeling bad, just accept it. Then repeat steps 1-3.