You say you're halfway though so im assuming you're well into your 2nd year. If its anything like adult nursing 2nd year then i know how horrible that is, alot of the time i felt like i wanted to pack it in and really doubted why i was even on the course. To be entirely honest im not sure im a nursing person either, i dont particularly enjoy the placements and often wonder if its something i could do for the rest of my life, then again is it just because havent found the suitable area to work in.
You say you liked the physical side of one of your placements, have you thought about training as a adult nurse when you qualify? Be dual trained, if thats something you can do as adult and MH.
Its not fair for your family to react how they have, its your course, your career choice and you've to do what you feel best. Cant stick with something you dont overly enjoy because other people think thats best.
MH nursing isnt something i know much about so not sure what else you can do with the qualification but you could look into it. Maybe you could go down the route of research, or maybe qualify and then look at more psychology related masters or something.