Why do I feel like I am using daydreaming to numb myself mentally and emotionally. ?

Daydreaming could be what you’ve taught yourself to do as a self-defense mechanism. Every time you’re in an environment/situation you don’t want to be in, you may be using daydreaming as a tool to disassociate. That might be a possibility. 

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Because you are.   Very astute. 

There's something I hesitate to mention because it's not clear from what you say that this is a problem with you. There's a lot of talk these days about maladaptive daydreaming, defined as chronic daydreaming that interferes with activities of daily life. It can be a serious problem. It's not officially recognized as a disorder and there's no standard treatment for it, although some people are getting therapy for it. An expert suggests going to a good online support group such has Wild Minds Network. 

A very simple way to answer your question is to say that you have a pain. Obsessing about things in the past is sometimes a sign of depression. You might want treatment for that. Depression screening tests online -  CESD R and for teenagers KADS 6-Item. I have advice from experts in my answers, about getting help, standard treatments, and self-help. I'll mention that you can't go wrong with self-help - it's something we all need.

Treatment usually begins by seeing the GP, who can give you a referral. I mention referral because just a bottle of pills is not a very good approach. The things you'd want to tell the doctor are how you feel at different times of day, any symptoms you might have such as change in appetite or sleep, and things in your life affecting how you feel.

If you're depressed, I can't tell you exactly what you need. There's no one size fits all solution. I can tell you though that there are healthy lifestyle choices that can enhance the effects of the standard treatments with office visits.  This has details about self-help based on cognitive-behavioral therapy and lifestyle choices that help with depression - nutrition, basic lifestyle things like sleep, exercise, and social support, also traditional Asian methods.

https://www.youtube.com/watch?v=wqEM_jlDRZI

My advice is find a therapist you can talk to about it, it may be more than just "daydreaming".