Hi Shaz51
Maybe this can help you -
My daughter was suffering anxiety about going to school each day (long story behind it). I put together a gift bag for her that I keep in the boot of the car. Into it I throw items from sale bins that appeal to her. It might be a lip gloss, a Freddo Frog, a hair clip, a greeting card, a fancy or funny pen, etc. I have also put the odd bigger ticket item in there, like a movie pass, or pedicure at a nail bar. Each day when she gets to school, she has to take something out of the bag, as a treat for turning up, late or not. If she is faced with a test that day, or handing in a late assignment, or something that ups her anxiety level, she will reach for the bigger ticket item to help her over the hump.
The point is to overturn the negative association she has made with attending school, and replace it with a pleasure response of some sort. It's no secret. We discussed it before hand, and with her completely in the know about the objective, it has still worked.
Whatever the cause of your negative association with Sunday afternoons, maybe you can work out a plan to consciously over-ride it, and reclaim your day.
Be well.