You wrote that you're generally good at communicating with each other. Well, this is an excellent opportunity to put those skills to good use. Talk with your BF about this. It's the only way you're going to find out why he wrote this, what's on his mind, & what it means for your relationship.

The worst thing you can do is to keep this to yourself & allow it to fester. Solid long-term relationships are built on good communication skills. So if you're thinking of this relationship as potentially being of the long-term variety, this is a great opportunity to build on what the two of you have already accomplished. If by some chance you see a therapist, this might be something to discuss with your T before you have the discussion with your BF.

Avoid being confrontational. Do your best to not allow the discussion to devolve into an argument. This is simply an opportunity for you to understand your BF's thoughts & for your BF to understand how reading this made you feel.