What to respond to my coach's email?

My cross country coach sent me a really long email about how he was disappointed in my focus on the sport... and as I read the whole email I started bawling crying because he also basically insulted me to my face like 5 times saying I was lazy and that my attitude is unacceptable . He said that he was disappointed in me like 5 times for not having looked at the chart he emailed us that said what zone our heart rates were supposed to be in, and also he said that I "need a major wake up call" and the whole email I guess was somewhat necessary because I have been slacking off a little (not in my workouts though, in other things like wearing my heart rate monitor and such) but it was really really mean and it made me cry for at least an hour. I read it about an hour ago and I am still crying. I don't even know what to reply to him. Do I try to explain myself, or just say I will work harder from now on? Or do I might as well quit the team bc the email was so mean? Like I really do not know what to do.... the email was such a surprise. Has anyone else ever had a similar situation where the coach like corners you and tell you you aren't focused enough? helppppppp