Dont sweat it North,I played nothing but core tdm and domination back in ops 1 days then I joined CAG just after ops2 dropped and was introduced to hardcore.
Hardcore was completely overwhelming for me in the beginning but I stuck it out and stayed persistent with playing hardcore game modes.I soon learnt CAG's way by slowing my gameplay down,holding back and letting the enemy come to me disregarding my score and never crossing that 50 yard line unless instructed that I was ok to do so.Since then I cant find myself adjusting back to core as i feel ill equipped and generally have the mindset of hardcore.
Also in hardcore lethal score streaks is something that everyone has to master no matter how good a player you are.It ain't an easy feat to drop a lethal streak and avoid teamkills(some streaks easier than others).I normally run non lethal streaks in hardcore.