I think the list of things that can make a relationship fail is probably long and varied and very different from person to person, but I'll offer up my two cents.
One of the biggest challenges for me has been the ways in which my husband and I have both changed over the years, sometimes growing together and sometimes as individuals, both of which are healthy and normal, but navigating a relationship during those periods can be difficult. The dynamics, expectations, and what the other person can bring to the relationship are in flux during those times and may end up on something different than where you started. There are times when I've felt disconnected, held back, or left, behind, maybe even a little unloved or resented, and I know he's had times where he's felt the same way. Again, I think it's normal to have periods like that in any 17+yr relationship and we've weathered them well (in general) but I can understand how some people would not.
I think this is particularly true of people who enter into these relationships at a younger age. As much as I thought I did, I don't think I truly knew "who I was" until I was closer to 30 and had enough life experiences to really figure that out for myself.
Another factor I see often, though it isn't an issue for us, is that I see people relying too heavily on that one relationship or giving up other relationships for it. I know a lot of women who think that because they are married, they can't have male friends, or people who think they can't go out and enjoy an evening with friends unless their partner comes along. Sometimes these assumptions are wrong, sometimes they really would upset/anger the other party involved. This, to me is wrong. You don't build trust in a relationship by never extending it and you don't grow as a person if everything you do includes a plus one. You end up with an odd sort of co-dependence and isolation (the fact that I see this happen so often could be somewhat cultural due to the region of the country in which I live).
So for me, the two biggest realizations have been that:
1. I'm in this relationship because I want to be, not because I have to be and that I need to be happy with myself before I can expect someone else to be able to live with me happily. I need to be allowed to be my own person and explore that and my partner needs the same. We both need to be able to form meaningful friendships with other people on our own terms, not dictated by the other, but also remaining respectful of them.
2. Changes in the dynamics of a relationship over time are perfectly normal and healthy, even though they can, at times, be really hard. Maybe it's not what you thought it would be when you signed up for this, but it can still be pretty great, and 5, 10, 15+ years of shared history, experiences, love, and support is something worth sticking around for and working on.
As for whether or not I am fully satisfied with my current relationship, yes, I am, very much. I get a little annoyed sometimes at some of the sacrifices I feel I am making because of my husband's current situation (e.g. I really want to move to another part of the country, but it's not possible right now due to some commitments he has), but the relationship is completely worth it and isn't suffering because of it.