It seems to me that balloons are your hopes/frustrations etc. and maybe you flying on them together symbolizes the sense of trying to 'rise above them'. But because they sometimes are 'unbearable', yours pop. Maybe this also shows the dynamic of a T-patient relationship. She is there to lend you her 'strong balloon that won't pop' during this journey of yours. Her lending you her bra, I think symbolizes your need for her to be maternal and nurturing.

The fact that she's saying 'we are secretly falling in love, let's not let anyone know' and that this fills you with dread, maybe shows something very common for all of us being in therapy: while we so very much seek a deeper connection with our Ts, their love and acceptance etc., when and if this starts happening, we feel guilty and shameful. Because we never got it and we feel like we don't deserve it (even though we desperately want it?) Or because it feels as we are 'betraying' the people that raised us and our painful way of relating with them, no matter how hurtful and even abusive?

Just some random thoughts; hope they helped! Will you share your dream with your T? I think you should!