I am on this site as looking for any helpful university info for my daughter but I saw your post and just wanted to reassure you that it's OK if you have days where you feel a bit down, especially at this time of the year when all the celebrations have finished.
However, try not to let it take hold of you. Think about what you would like to get out of your year ahead, write down your professional and personal goals. Break them down into mini goals so they are achievable. Striving to meet goals will help your well-being and hopefully keep those blues at bay. If not, don't be afraid to reach out for help - tell yourself you deserve a good life and to be happy! J.x