Why are you on a low dose of an SSRI? Generally it's better to increase the dose of the SSRI you are currently taking and if that's not sufficient, or perhaps switch to an SNRI or add/switch to Wellbutrin.

If you want to try an add-on natural supplement I suggest fish oils instead, which have other health benefits too. If you want suggestions on more specific info let me know. Good non-med options include therapy, meditation, and exercise.