This weird, detached feeling sounds to me like you're dissociating. For me, my brain goes numb and/or I get detached in order not to feel/experience painful/strong emotions that I wouldn't be able to handle on my own. Seems logical to me, that you'd feel like this after your session with your therapist, especially if you talked about difficult topics and/or the session was emotionally intense in other ways.

Also it makes sense in a way that in a situation like this you'd turn to drink/self-harm if these are your established coping mechanisms.

Talk to your T how you feel afterwards, about feeling dissociated, numb, detached. Ask her/him, if you together can come up with ideas how to better cope in situations like these? For me, the dissociation quite often would start in session, sometimes head on, sometimes quite subtle. My T was quite good on picking up on the signs. So quite often she would do some grounding exercises with me at the end of intense sessions. Once or twice when dissociation hit me pretty hard, she'd asked me if I wanted to sit for a while in the waiting area in order to regroup. Which was helpful for me, so I could come round in my own time and didn't have to go outside in a pretty dissociated state. And she'd asked me what I would do afterwards and/or this evening. So having a plan in place (like taking a bath, watching a movie, do some nice cooking, having a friend come over etc.) helped me to cope in more healthy ways.