Broken engagement

Has anyone ever ended an engagement? How did you know it was time to leave? My fiancé and I have been together for almost 4 years, and our relationship is just going downhill fast. We both are always irritated with each other, I feel like everything I do pisses him off, and I’m just tired of crying about it. I’m in med school which is really taking up most of my time and I try to still be active in our relationship but I don’t have as much free time as I did before school. Long story short I just don’t know what to do. Any advice?