Antonyms for the phrase "am a foundation for" could include terms such as destabilize, undermine, weaken, or obstruct. These words all suggest a lack of support, rather than the solid base and strength that comes with being a foundation.
To destabilize something means to make it less stable, while undermining suggests actively working against support. Weakening implies a reduction in strength or durability, and obstructing creates a barrier or obstacle that prevents something from being supported.
While being a foundation provides a strong basis for growth and stability, the opposite can create insecurity and doubt. Therefore it is important to ensure that our actions and intentions align with building and strengthening, rather than tearing down or dismantling.