We all have bad days. Days when we feel low, ineffective, perhaps unappreciated.
There are a few things to remember on such days.
Being nice to yourself means surrounding yourself with the things in your life that tend to bring you up, rather than down. Schedule meetings with people whose ideas you find stimulating, and save the challenging folks for another day. Have lunch with a supportive colleague or friend. Listen to music that lifts your mood (a link to my favourite 'bad day' song - a video - here.) Read something that inspires you, and save the newspapers and articles focusing on what's wrong with the world for another time when you feel a little more resilient.
Being nice to yourself includes giving yourself permission to wallow in suffering for awhile. Just telling yourself that you have nothing to complain about doesn't actually make a difference. How you are feeling is how you are feeling. Remember the adage 'what you resist, persists'. If you are really successful at wallowing, you might even be able to see the glint of humour in the drama you are creating about your life. Humour goes a long way towards moving through the low feelings.
Something else useful to remember is that no matter how bad things seem, this isn't how your life is. It is just how you are feeling now. When you have moved through it, you will likely have forgotten how you felt, as well as what took you there in the first place.
Lastly, you can remember that you get to choose the point of view you have, the one that has a large bearing on your mood. If you decide to focus on what's wrong, that is likely what will show up for you. If you instead choose to focus on the things that are going well, you will see more and more of those things in your life. And that perspective tends to feed on itself.