You will die a lot. That's part and parcel of an MMO. The penalty is very mild in TOR though - from what I can tell it is just extra equipment wear rather than an XP hit.
As far as spawns go, always move away from a mob's spawn point as soon as you have dealt with them, lest you experience exactly what you did. There was some discussion of respawn rates in another thread. They are a bit variable, depending on the level and the planet/area. The more populated areas are likely going to have very short timers for the trash mobs, so it may pay to stop and ascertain what that rate is for a given area before wading in.