News flash: there is no "One," a perfect being that is going to do all and be all for any of us. That is a story for a great fictional work. There is that person who might be attractive (and that fades fast along with the niceties). The individual is flawed. Like you, he or she makes a choice to either stick it out with you and your "issues" or not.
It is quite easy to get along in a relationship initially because two people have never hurt one another and are just getting to know what buttons not to push or else ruin something they are starting to build or have been for quite some time. But as the relationship gets older, the pair get quite comfortable saying and doing "whatever" without too much thought or care about the damage actions might cause in the short and long term. There really is no single reason as to why things change in a relationship, they just do whether the influence was external or internal. You either have the energy to weather the storm or you don't.
The one mistake many couples make is to go all the way back to the good days and say things like, "How I never should have...Why did I? I knew he wasn't right...She was trouble from the start!" Well you slept with that person, dined with him or her, and did other things, so if he or she is a fool then what does that make you for keeping the individual around? What current issue is going to be solved by looking back and regretting your every move with a lover/spouse while tarnishing what good there was or still is?
Live for today, recognize what is happening now, and work toward change within and around you.
On a side note, sometimes the person you are disputing with is the one that is meant to help you mature personally and professionally. He or she may not be forever and always, but you do learn a lot about you good, bad and otherwise.