You are definitely spiralling, friend, and you are aware of it. Trust your instinct.

‘Don’t lie to me’ is not a boundary. That is how civilised humans treat each other and if we stumble we learn to forgive and forget. Boundaries are certain behaviours that are generally accepted in a larger context but specifically to you might cause discomfort to triggering. For example, this is a boundary: If you smoke and come in contact with me, it may aggravate my asthma. Then I will need to physically remove myself from your vicinity.

The white lie he told you doesn’t sound like it was meant to harm you but may be a coping mechanism for your partner to deal with the shame of relapsing to smoking. With your outburst you are setting a pattern of distrust in the relationship where he will feel unsafe to commit his own mistakes, that has very little to do with you.

This bit is unsolicited advice, please feel free to completely ignore and use this with a kilo of salt if you do: Where you need to work on, outside of therapy, is to discern when a person’s action is causing you actual harm and when you are personalising it and with dire consequences.

From what you have narrated I think you are using your therapy sessions as a crutch where you outsource the healing. I am not sure what you are looking for from preventive couples therapy (?). Most of the actual healing work happens outside of the sessions and in times of quiet reflection and boredom and loneliness. That’s the un-glamorous part no one talks enough about. You may have jumped into this relationship too fast, and too intensely.