I can't speak to whether you should tell him about it or not.

But, this is really good learning/growing material for you. You felt so utterly overwhelmed and devastated that you had fully planned what to do about it. But in that plan you gave yourself a chance - you were going to wait until after Monday's session.

Then Monday's session came and it was better and you felt better, and the overwhelm passed and you were able to abandon your plan.

This is the learning and the change right here. When those devastating feelings overwhelm you, try to observe the cycle of what happens... because the cycle is they will overwhelm you, and then they will go. They will ALWAYS go. Even when in the middle of them it feels like nothing will ever exist again except for what you feel in that exact moment.

The more you can learn about that cycle and hold on to your knowledge of that cycle and keep a part of yourself detached and observant and waiting, the more stable and even-keeled and growing you will become.

Has this cycle happened before? Does it always follow a similar path? What things bring you out of it and back to even-keeled and rationally-thinking again?

The key to ending or healing this cycle and reducing the danger from it is in understanding it and bit by bit, developing the ability to hold on to the knowledge of it through the hard times.

In my experience being able to discuss this cycle with one's therapist is actually really helpful too. Sometimes when overwhelm is triggered the therapist can prompt or remind the observant remembering part of self or "wake them up" or activate them to remind them. Then that part can step in and hold the safety until the overwhelm subsides. It can become part of a shared understanding of a safety plan.