This has totally been an issue for me for a while. I thought it was just me!
I am now lvl 40 and play a Consular Shadow Tank as my main. I am now at the stage where I am desperately trying to not kill ANYTHING or achieve anything (outside of class mission) out of fear my lvl 34 and 35 Balmorra missions will grey out. I am now doing NO bonus missions also. This has been building since lvl11 when I finally left Tython... I have lost a few mission to the "greyness" and a flashpoint or two but now I will probably be arriving at the next planet already fully greyed out because at this rate I will be 7 lvls over!
I thought everyone liked to do all missions, all heroics, all bonuses, explore on each planet, kill any enemy standing, look in each room/cave/corner and do all available space missions and flash points. I don't double up on anything though, nothing. I just wanted to taste all PvE content and it's kicking me in the a.s.s. now. Clearly Bioware planned the XP gain / leveling progression for the majority of player trends, which it seems try to get through the class story asap and do a some other missions here and there, but not 100% of everything offered, let alone actually explore "sandbox" style.
I'd love an XP lock (but keep getting credits and reward gains) so if I felt like hanging around a planet and exploring I could do so without fear of eventually being so far above lvl that I eventually arrive at a planet with potentially everything already greyed out. I guess my next 10 lvls will be just sticking to class story so I can balance back in.