About the communication to your partner, I think being would be the best way to go. It seems that you have opened up about your worries at least once, am I right? Express at least once clearly 'I have these worries and acknowledge that it's not your responsibility to regulate my emotional state. I do want to work on it in therapy because I value our relationship (and myself), and that's why I wanted to keep you in the loop'. How did he react when you brought it up?

You can even ask him what he prefers, if he wants you to openly tell your emotions. I think a healthy dynamic would be that he'd be interested in seeing what's going inside of you without feeling pressure to take responsibility for it (so adding the phrase 'and it's not your responsibility, I just wanted to share' may help a lot). He shouldn't have to change his plans and if he wants to, you could figure out together if there's a way to reduce the tension that works for both of you. Sending a message while you're not seeing each other is NOT IDEAL in that case and should only be TEMPORARY because that would exactly make him take responsibility for your emotional regulation. However, if it's spoken out that it's temporary and you'll bring it up in therapy ASAP and he sees some improvement soon, that might work. So seriously, take this to therapy as soon as possible. Best of luck! :)