My Level 45 Sith Marauder loses health pretty fast in combat. I don't know if it's because of my equipments, weapons, or if I'm just bad at the game. To me my weapons stats are looking pretty nice, so it might be the equipments stats? I would greatly appreciate some tips on how to solve this problem.
I suspect you're either on Voss or completing the Hoth bonus series.
In anycase... towards the end of the game, most MOBS mainly do unresistable damage (internal/elemental). You take 100% damage. Even with 10K hitpoint, they can burn/poison/bleed/shock/etc. you to death very, very quickly.
You kinda get an idea of this as early as Quesh, or Tatooine - Czerka lab infected mobs.
I don't think you're doing anything wrong. Not a lot you can do, except the usual and just have the brute force it.