Hey there, I am working as a psychologist in the area of performance psychology and mental fitness. What I realized about people feeling held back by self-doubts is that they have internalized certain limiting beliefs.  
These can be beliefs that are rather paralyzing such as "I am not (good, talented, experienced, ...) enough" which reduce motivation and are often accompanied by the belief that we cannot change anything about the situation. And then there are limiting beliefs that make us speed up, but also ignore our own needs, such as "I need to be perfect" or "I need to be strong".  


If we tell ourselves these stories over and over again, they become a mental habit. But thanks to a process called "neuroplasticity", our brain changes with experience and we can also strengthen positive and empowering beliefs over time.  


So in order to let go of self-doubts, ...  
1) start by identifying your limiting beliefs: Which thoughts cross your mind when facing a difficult challenge? What convictions or stories do you tell yourself over and over again?  
You can combine this with a **"free flow of thought"** journaling exercise (setting a timer for 5-15 min and writing down everything that pops into your head without lifting the pen from the paper - even if it doesn't make any sense) or with a **guided meditation** (there's for example a session called "Leap of Faith" in the Flow Lab app that aims at becoming aware of limiting beliefs).   


2) Define an **empowering belief** that you want to tell yourself instead. Formulate these affirmations positively and in the present tense (e.g., "I've got what it takes" or "I am worthy because I stay true to myself").  


3) In order to actually believe this new empowering affirmation, formulate yourself so-called **"guiding questions"**. These are questions you can ask yourself in a giving moment to change your perspective for the better, such as "Which similar situation in the past did I master?", "What can I gain when tackling the challenge?" or "Which strengths and resources do I have?"  


You've got this!