Formal:
* J'ai pensé que vous m'aviez oublié. (I thought you had forgotten me.) This is the most literal translation.
* J'ai cru que vous m'aviez oublié. (I believed you had forgotten me.) This emphasizes a stronger feeling of uncertainty.
Informal:
* J'ai cru que tu m'avais oublié. (I thought you had forgotten me.) This is more casual and is used with someone you know well.
* Je pensais que t'avais oublié de moi. (I thought you had forgotten about me.) This is even more informal and is typical of spoken French.
Adding Emphasis:
You can add emphasis to any of these sentences by adding:
* un peu: (a little) "J'ai pensé que vous m'aviez un peu oublié." (I thought you had forgotten me a little.)
* complètement: (completely) "J'ai pensé que vous m'aviez complètement oublié." (I thought you had completely forgotten me.)
Context is Key:
The best way to choose the right phrase depends on your relationship with the person you're speaking to and the overall context of the conversation.