More often than not, lack of self-trust is rooted in low self-esteem. While it does take time to build your self-esteem (there'll be posts to help with that in the future), you can actively work on building self-trust from today.
9 Signs You Don't Trust Yourself
1. Needing other people to co-sign your ideas before you make a definitive decision
2. Not keeping promises that you've made to yourself (often because you're trying to please someone else or for short-term gratification)
3. Ignoring or minimising your needs and true feelings
4. Constantly questioning the decisions that you make, even if you don't have new information suggesting that you might've made a wrong decision