Use: “I want to lose weight.”
Do not use: “I want to lose my weight.”
Use: “I lost weight.”
Do not use: “I lost my weight.”
Treat “gained weight” in the same way.
Say: “I gained weight”
Do not say: “I gained my weight.”
Since talking about your health is a good way to approach a lot of the TOEFL independent essay questions, I see the above error quite a lot.
I totally understand the error, as we do say things like “I lost my hair” and “I hurt my back.” Just remember that “weight” is not a body part!