Rough transition

I am having a hard time adjusting to being back to work full time. I feel that I am not doing a good job of being there for my kids and that I am also not doing the best job that I can at my job. I broke down crying tonight in front of the kids after a stressful bathtime experience. Reese sweetly asked me what hurt so she could kiss it to make it better! So sweet I only cried more! How do you explain to a 3 year old that sometimes you just need a good cry?