Thoughtforms can only attach themselves to your mentalscape, they can’t “physically” drain from you like a real entity can, thoughtform parasites feed off you through means of your attention be it fear, praise, love, hate, etc.
A entity parasite, for example these ghouls that wander the etheric plane, they cannot speak, they’re practically purely instinct, they feed through contact i.e biting and latching onto you (despite them varying in size from small dogs to large bears)
some parasites use tendrils to latch on to you, but the common thread is ‘most’ parasites are incapable of the same level of thoughts as the common man and are mostly instincts and animalistic.
You can kill it yourself if it’s a legit entity or you can petition for a Deity or Demon, or Angel, or any race you know for help.
If it’s a thoughtform you work to keep it from your mind, through shielding your mind and forgetting about the thoughtform.