Y'know, I think it really boils down to the fact that you have to give yourself permission to value yourself. That's something I've definitely struggled with. Especially as a super busy working grad student single mother, I know I have a habit of putting everything else before me. In the last couple years I've worked really hard on valuing myself more and making myself a priority. That's meant trying to establish regular exercise even if there are dishes in the sink or a zillion pages of class reading to knock out. It also means trying to eat in a somewhat healthy manner. I haven't been perfect about it (I've been doing the 1000 minute challenge for like four months and haven't come close to hitting the goal) but I have been working at it consciously. My next semester is going to be a little insane -- I'm taking 9 graduate hours and working 20 hours because at that point I get full benefits (!). I was just trying to figure out how to schedule in exercise time. I think that will just have to mean dropping DS off at daycare at 7:45 rather than 8:45 two or three days per week, or possibly working out late in the evening after he goes to bed. It's not ideal, but that's the reality of life as a single working mother.
To me, it boils down to this: I deserve to be healthy. I deserve to feel strong. I deserve to eat food that is good for me rather than harmful. And damnit, I deserve to feel happy about my body. And of course, it also sets a good example for our kids.
I would say be gentle with yourself, but sit down and map out a plan. Schedule in exercise time just like you would schedule work or study time. Meal planning can also help tremendously. Buy healthy things for quick meals like beans and rice, big salads, fish, that sort of thing.