Just one thing? I feel like there are often multiple issues that lead to a break up, relationships just aren't usually that simple. I will do my best to tackle what I think the biggest issue was.

I think the most important thing would be to maintain a respectful manner of communication even during challenging times. This is perhaps one of the greatest challenges for anyone in a relationship, and the longer you are together the easier it is to stop working actively at that. It is so important not to lose respect for the person you are with in order for it to work, and once you do it is very difficult to come back from that.

When your partner does not speak to you with respect, that often sticks in your mind. You remember those moments. It makes you feel that they never seemed to really love you at all by their actions and words, and it takes such a toll on the relationship. Name calling, tally keeping, being jealous over time spent on hobbies or with other people (family, friends) are all forms of disrespect or selfishness that can cause your partner unhappiness. If we had a disagreement my ex could get so nasty and hurtful. It did such a number on my self-esteem and when I look back on it compared to my relationship now it is hard to imagine that I endured that kind of behavior for as long as I did. But had I not gone through that I would never have learned what it takes to build a long lasting and successful relationship.

I have been with my current partner now going on 9 years and I think we are still happy and together because of this. When stressful times hit and we have to have difficult discussions or make hard decisions, we work actively not to let the stress and frustration cause us to disrespect each other by yelling, name calling, etc. We always try to speak with one another reasonably and work on solutions collaboratively without hurting one another. If we find our tempers are too high to discuss a sensitive issue reasonably we will usually give each other space to work at a hobby or unwind to give us time to gather our emotions and calm down before trying to talk about it again. We make a point of not holding discussions if one or both of us is feeling too upset or emotional to have a rational discussion.

Nobody is perfect and of course there are moments where someone says something hurtful, but the important thing is to be able to recognize when you said something unfair or hurtful, own up to that, and make amends. If my ex had done that, who knows? But if he had to change that much, would he still have been the same person in the end?