im 14 and throw 65-67mph fastballs on flatground. does this mean i would throw harder off a mound? ive developed everything i have off flatground and just that one time took it to the mound and felt much more comfertable. will pitches break more?

Its different for everyone. I can hit (just hit not throw consistently) 86-87 off flat ground and hit 82-83 off of a mound. Really just depends on your mechanics and what you are used to