What are some more proof that shows that Katniss was not reluctant in volunteering, apart from the fact that, Katniss stated, "it hadn't mattered"?

Because, it could be an "assumption' that nevertheless, she was going to volunteer, what proof however, do we have to back up this assumption

Also, the fact that,nobody would really volunteer from District 12, considering its your own death sentence, did that somehow make Katniss think otherwise of volunteering for Prim, could you provide evidence, thanks.