While there is a lot of good advice in this thread, much of which has worked with me (paleo, activity vs. exercise, etc.), there is another factor altogether that we haven't discussed.
You are specifically asking about motivation. In my experience, you cannot force yourself to be motivated. Things in life have to line up. When I am struggling with my eating, when I cannot seem to get motivated to do my workout, when I'm not sleeping well, etc... it means there are other things going on in my life, different possible types of stress, that really need my focus before I can get on track. I only have so much energy.
Right now, I'm going through this - while normally a very active person and a very clean eater, I have not been able to motivate myself to workout, and all I want to eat is crap food. It doesn't matter how much I know this is bad for me...I got some shit going on. I am going through a massive amount of professional stress, have been interviewing and figuring out what I want to do with my life, and all of my energy is going to get this sorted out.
Based on experience, once I "fix" that problem, get back to an even emotional keel, the rest all comes back and maintaining a healthy life style is, well, not effortless, but doesn't feel like so much work.
If motivation is your issue, I'd look at other areas of stress in your life and address them. If you have a new baby...well, there's a glaring red flag that you've had some major life changes, lol.
(The catch-22 is that regular exercise and healthy diet can help you deal with stress - but sometimes you just gotta focus on one thing at a time and cut yourself some slack if things have gotten too out of hand).