I think I've messed up a bit while leveling. Right now I'm level 38 and just finished up all the quests on Hoth. I wanted to move to Belsaris, but when I get there everything is level 42 to 44 and quests are red. I didn't start doing bonus quests until Tatooene because I ran into something of a similar problem. I also haven't been doing the heroics or flashpoints since I can't seem to get a group for them. Do I need to go back to Hoth and grind on mobs for a while until I'm around level forty? I've looked at some leveling guides but I've already done the bonus quests on most of the other planets like Taris.

I'm not sure how this is possible. Doing the quests on every planet should put you over level not under. I'm level 42 and barely touched Hoth. Two things that could have a negative impact on xp earned would be if you are able to stealth past many fights, and if you never ever have rested xp stored up.

Rest doubles the xp you get from killing mobs. Stealth prevents you from getting any xp at all, and also robs you of bonus xp for completing the bonus missions.

There are some 38-40 zones on Alderaan. If you don't see a quest in the starting area, buzz around on your speeder in unexplored areas.