Dated a fellow ISTJ for 4 years, then broke up. We analyzed what went wrong to death. We discussed what changed in different times during the relationship and how we could have changed our behavior or acted better during crucial times. In the end we just weren't the right life partners for each other.
I haven't really gotten over the relationship yet. Someone told me that it takes at least half the duration of the relationship (in my case, 2 years) to fully recover, and I'm sure it'll be awhile before I can even have a normal conversation with the guy again.
I don't do the casual dating/fling/hookup thing either. If I care about someone, I really put myself into the relationship. If it's a one-time date that doesn't work out, I don't think about it much.