On Separations. And sunk costs.

Separations.
When you think about couples getting divorced, you hear about what happens to the kids and the effects, etc. But one thing that you rarely hear about is the extended family of the couple. Presumably, both members of the couple have established some level of relationship with each others' parents, siblings, and even close friends. So what happens to those relationships when the couple breaks up? (The same questions apply to a regular break up, not just divorces in married couples). Do you lose those close relationships with the people your partner is close to just because you are no longer with the person? I would say, the natural answer would be yes. But that's easier said than done I think. How do you just cut ties with people who you have invested time, energy, and love for any number of months to years? I wonder this with regard to separations of friendships too... I've had this experience a few times now where a close friend and I have a fight. In many cases, I was also close to the person's family, so naturally I no longer got to see those people when my friend and I were fighting. But then, not only did I miss seeing my friend, but I missed seeing their family. Obviously, I couldn't just call up my friend's parents and show up at their house. But I wanted to... it has been experiences like this that make me wonder (and a little... no, a lot... scared) about divorce.

Sunk Costs.
This is actually on somewhat of a related note... but another question on my mind is about sunk costs. I started thinking about it in terms of friendships/relationships, but I've realized it applies to a lot of other areas of my life too. First, the friendship/relationship aspect: if things are going badly with a person you have invested a lot of time/energy/emotion in, when do you draw the line and give up? The natural inclination (at least mine) is that I've spent X years on this friendship/relationship, so the longer I have spent, the less I want to just throw it away. The more I want to try, even if it continues to not work. But when do you stop trying, and consider all the time and effort a 'sunk cost?' When are you losing more than all of the time/energy that you have already invested is worth?

Like I said, this also applies to other things. I'm thinking specifically of a few projects I am involved in. I've devoted months to analyzing data and working on various aspects of various projects. After so much work, some of the projects have turned out to have very little in terms of findings. But there is still the possibility of something IF we continue to work on it. I am so sick of working with some of the stuff, that I have no interest in working on it more now. BUT I have already invested all this time, and currently would have nothing to show for it if I walked away. On the other hand, if I invest MORE time, I might end up with something useful. But it'll require even more time and energy on something I don't want to be involved in anymore. Where do you draw the line and chalk the time up to a sunk cost, and scrap any future involvement? Or when does that sunk cost become worth sticking it out... potentially endlessly??

I have no answers on these matters, only questions. So, opinions are welcome.