LOL: It makes me laugh how many blokes think this is a blokes-only forum!
After graduating me and my then GF (now wife) lived about 3 hours travel apart and I recognise the Sunday-afternoon stress thing. I still remember it nearly 20 years later! From my point of view (i.e. a chap) it came from the ‘about to depart for the week’ thing and another 2 issues:
1) I was independant and living with people who were able to be entirely spontaneous at the weekend, whereas I always had my GF coming to stay who just wanted me for herself. At first I didn’t mind, but it can be difficult being with a cliquey group all week when they’re laughing and joking about things that happened when you weren’t there.
2) By Sunday afternoon I had spent 35 hours solid with her. Even these days I probably only get that in a week! (excluding sleeping). The difference in the cycle of all-week-without-her, all-weekend-with-her was almost too much!
I suggest you keep the above in mind – the reason for Sunday afternoon stress may be more than just about the parting of the ways for the week.