I always have two mindsets going into a relationship:
1. I honestly believe this girl and I are almost destined to be together
2. I'm unsure of how it'll play out but I look forward to finding out
Now, my last relationship was one of the latter and we had a great relationship, got along really well and had fun but, at the back of my mind, I always wondered whether I'd ever marry her etc and, in the end, I broke up with her after a year-ish
My relationship before that was with one of my best friends. We had been mates for years and eventually got together. Because of how close we were and the way we got together (and the fact that she was gorgeous), I really thought we were made for each other. However, as it turns out, she was a total prick and we eventually broke up after eighteen months of having a very bi-polar relationship
Now I have a huge crush on a girl (and she knows it) who has never had a boyfriend and has built up this perfect Disney view of relationships and I think she's scared of being with me in case she's settling and her prince charming is out there somewhere... but she's coming around, I know it (because I am awesome!)
EDIT: Not sure I actually answered the question... um, I guess my second category is the closest I've come to settling but I wouldn't class it as settling, just being less sure about the relationship