Hi! It's great to hear from you.
I agree with you that learning about your past only helps you understand where your feelings come from but it does not help you change what you feel now. For example, you can see that low self esteem comes from a parent that put you down all the time, but that understanding does not change the fact that you have low self esteem now and need to address it.
In order for real change to occur, you have to loosen the fixed ways you are thinking about yourself. The issue here is how to change thoughts that you have become used to over the years. Changing those patterns is key to helping you increase your self esteem and lessen your reaction to what others say to you.
Building your self confidence is a matter of deciding to do so. It takes some work, including learning more about self confidence and applying what you learn. Here are some resources that can help:
By learning more about self confidence/esteem, you can give yourself the tools to change your thoughts. You have to stop the "learned" thoughts you had from whatever source they came from and make your new thoughts your own. In other words, you are giving yourself the messages that you should have gotten when you were younger so you can build your own self esteem.
When you suffer from lack of confidence, it is going to magnify anything around you that fits that pattern. So when others criticize you, it will go like a hot knife through butter right to the root of your feelings, confirming your thoughts about yourself. In order to change this, you have to have a counter thought you can use to tell yourself. For example, if someone questions your ability to do something, it helps to have a comment you can tell yourself (or even that person) that confirms the opposite of what you feel. If the person says, "this is hard for you, isn't it?". That hurts. But instead of letting it confirm what you feel about yourself, choose an opposite statement such as "It is hard now, but I will learn it and be good at this soon".
Make a list of affirming statements about yourself. This can be difficult since you probably tend to think negative thoughts. If it helps, ask others around you like your partner or trusted friends to help. What are the good things about you? What would you like to improve? What things are you most proud of doing? Etc. Make this list, look it over often (post it somewhere to read it each day if it helps) and add
to it as you go along. By doing this, you send positive messages to yourself, the ones you deserve to hear rather than the negative ones you had to hear before.
It can also help to have friends and family affirm what is good about you. Hearing it from others gives you another way to take those messages in and make them part of your own thought patterns.