As the saying goes around here, it's not about motivation, it's about commitment. Commitment is what makes you push through when motivation is nowhere to be found. And those times are many. What seems to work for me, after a relapse, is committing to just ONE on-plan day. After that I can usually commit to two more, and after that to the whole week. At that point some motivation usually kicks in.
And just to clarify: For me, on-plan is 1,500 cals per day to lose, 2,000 cals per day to maintain. Overrestriction never works for me, no matter how badly I want to "make up for" a relapse. It's almost a law of physics: push the restriction pendulum too far to one side and it WILL swing over to the other side.