Always Underleveled

You can be underleveled and do most of the side quests and bonus missions and even some bonus series. This happened to me but with another character it didn't. I think the main difference was I wasn't accumulating much rest XP and I wasn't using those +25% experience boosters (which didn't exist back then).

Well my valor level is only like lvl 20 I believe but even if I got it up all the way to 38 I probably still wouldn't even be lvl 40. Pvp offers nice xp but nothing that great.

I always log off in either the ship or the fleet. I sometimes use the xp boost.

This doesn't make sense to me, the OP's post doesn't make any sense either. I have leveled 16 characters to 50, 12 of them prior to the XP boosts becoming available, and it would be fair to say I have a good understanding of the leveling process.
Yes, rested helps, but only to the point of pushing you further ahead of the leveling curve, definitely not enough to swing you 2 levels behind by not utilizing it if you're doing all the activities the OP says he does.

Well its not like i would lie lol. What do YOU suggest I do then?

It still could be rested XP though... if you play a lot you won't get the benefit from it.

Simply, if you burn you rested xp and go to normal xp then log off for a few hours and come back it won't be long before your rested xp gets used up again and you are back to normal xp. It happened to me on my first toon. I played so much that I didn't get much benefit from rested xp because I was always playing.

The equalizer for me was flashpoints. I ran through them many times for gear and xp.

I speed level now and can be at level doing the class quests and the planet quests with little or no side quests or bonus. XP boosts are my friends.

That's incredibly odd. according to your playing habits description, OP, you should constantly be several levels above your class mission. the only thing I can think of is that a lot of times, you get 2 possible planets to go to - one of the appropriate level and the other - the planet you go to right after. but I think by lvl 40 it pretty much thins down to one planet at a time. so honestly, other then possible bug - I have no idea.

incidentally about subscriber questions? only subscribers can post on the forums, so anyone posting here has got to be one.

I'm overleveled w/my Scoundrel, but I tend to take as many heroics as I can get. Most are survivable, even with not-overly-good PUG groupings.

Would the fact that I skipped the taris and nar shadda bonus series have anything to do with it? I am really stumped. :/

On my latest char i did every class quest, bonus quest and side quest, including the heroic 2+'s which i solo'd but not that Heroic 4's. I did not do any warzones, space missions or flashpoints. I was so far ahead that once I finished Hoth I was able to immediately start the Hoth bonus series (translates to about 6 levels ahead). I can't see how missing 2 lots of bonus series (which are really only 5-6 quests each anyway) would translate to such a huge disparity. I have no idea how you are so far behind, as with everyone else this really stumps me. I would not suggest re-rolling. As others have said, I would do all the space dailies that are green or higher for you at the moment each day. They really do reward quite a lot of XP for the time invested and you should catch up pretty quickly.

EDIT: That said, you are probably in the unique position of actually being able to see Corellia and Voss properly, as everyone else usually hits 50 on Belsavis or shortly after and only does class story on Corellia