(((peanut))) i don't think you're "stupid". not one bit. you were hoping to explain something that is important to you (it doesn't matter when it happened ... if it's still bothering you enough that you bring it up now, then obviously it was important and is STILL important), so please don't berate yourself for this. it IS important and needs to be dealt with. peanut, it wasn't a waste to bring it up ... it was brave of you to bring it up. your dad is in denial and minimizing what you went through and the impact it had on you. maybe he's even trying to make himself feel better about it, but meanwhile he's making you feel worse about it.
i'm like TAS. i have told one other person about therapy stuff once, but that was enough. she made me feel like i was living in the past and totally minimized my feelings. i'll never make that mistake again.
meanwhile, peanut, if there's stuff in your past that is bothering you, then it IS affecting your present and for you to heal it is necessary for you to revisit it, and it's not a waste of money nor a waste of time. keep going, and please talk to your T about the exchange with your dad.
good luck, peanut. please keep coming here for support.