Why did your long-term relationship end?
I've had two relationships that I hung around for three years+. I've been married for far longer. Marriage is a different animal. The level of commitment is higher, and also, $$ is involved.
But I digress. You asked about relationships. This is a difficult question. I had to think about it. I suppose my answer is that it took that long for me to determine that I unequivocally did not want to marry my partners. Typically I know within a year if I will or won't. But a couple of times, it just took longer and ... yeah, I probably should have pulled the trigger much earlier.
In a torqued way, you can see the positive in this. I liked both men a whole lot, and our overall level of compatibility was high ... except for those pesky, nagging maybe-major deal breakers that I knew could change. Failure to work/self-support and on-off substance abuse, respectively. After sitting back and letting it play out, I acknowledged that it was in my best long-term interests to move forward.
I just wasn't gonna put a ring on it. I think that if you poll most people who end long-term relationships, they'll say the same thing.
This goes to User's answer. Relationships should not feel like slave labor. You should want to make an effort, genuinely, and cheerfully. When that is no longer true, it is time to call it a day.