What did just happen? And why does it hurt so much?

About 3 weeks ago I received a text from my then-bf of 1.5 years. He said in response to my texts that he was struggling with managing everything and he does not have a time for anything else than work and grad school. He said he cannot manage a relationship and told me to move on to find somebody who can give me time. What did I text him prior to this message? I asked him if he was seeing someone else or trying to break up because he was distant and off. I did not think it was because of him being busy because he was always busy with work. Sometimes we have not met for 1-2 months. We usually met once a week or so. We talked all the time though. I have never complained of him being busy because I genuinely understood him. We never had an argument or problem so this is much harder for me to grasp what the heck happened. He won’t respond to me. So being ghosted for almost 3 weeks. This is so not like him so it is harder for me to understand it all.