trust comes only if you believe you will be safe + secure. many people who experienced emotional, physical, or mental traumas in childhood (even mild traumas, which pretty much means all of us) occasionally struggle with trust because at some point they've interpreted an experience as meaning they are not safe. therefore, a way to establish trust is to work on all the ways you notice you are safe + secure.
garnet - safety + security issues settle in the root chakra, so balancing this chakra's energies are essential for feeling safe enough to trust.
carnelian - what if it's yourself you feel you cannot trust? many of use are likely to break promises we make ourselves than those we make to others, which can lead to lack of trust in self. lack of integrity (including with self) is a sacral chakra issue + the carnelian can balance this chakra.
amethyst - another thing people feel they can't trust is the universe in general. they may feel it's generally unsafe + act from this. amethyst helps you connect to divine guidance + following this guidance leads to good results, which leads to greater trust in the universe.