Issues with MIL, low confidence, negative thoughts

One enters into marriage with dreams of hoping to start a new journey with their partner that is filled with love and happiness. Having your relationship controlled by your in-laws can be frustrating and is certainly not why someone enters into a marriage. I can imagine your pain and agony.

For a happy and successful marriage, both partners need to be equally invested to work towards it. Speak about your concerns with your husband as well if needed. It's important to have your spouse support you at such times.

I understand your concerns and it would help for you to find an appropriate time to have an honest conversation with your mother-in-law about what's bothering you. While attempting to do so, it is important that you convey your concerns sensitively and not in a way that conveys blame, frustration or anger. That is likely to escalate things in an undesirable direction.

You can start a conversation by saying "I would like to speak to you about something that has been bothering me since a while now but was worried how you might take it. I feel disrespected and devalued when (explain with instances when you felt your self esteem was attacked). I long for you to value me and believe in me more".

The tone and choice of words matters a lot in a conversation and decides how the other might take it. Speaking in a manner as expressed above will reflect that you were being sensitive about her feelings and are open to considering her thoughts as well. Beginning by showing appreciation for her intention puts the air to ease and makes it easier to have what might seem as a difficult conversation. This will make her feel heard and also be willing to work through it with you.
Saying "You're always so full of negativity towards me!" is less productive than saying something like "I would really appreciate if I were given some more respect and valued in our home". Talk about what you want rather than what you don't want.

Start small. Take up one small step towards making one small change in one of the barriers you found to your issue. Gradually increase in addressing to other steps towards as many issues as possible. Remember, consistency is the key. Even if it is only one step that you all have decided to take on in a long time, it is okay.

I am sharing with you one article here that you might find helpful to your issue:
https://www.askmile.com/blog/in-laws-interfering-in-your-married-life/

With regard to your self esteem issues, it would help for you to speak with a counsellor or a therapist who would be able to work through these issues with you in an effective way.