Reading your concern makes me reiterate my childhood where I continuously asked this question: why does my face seem too big? I believe everyone has a phase where they go through a “slump,” losing all hopes and entertainment in their life. During this phase, you will typically feel that you do not like the version of yourself. You might feel lost, not even sure if such slumps will ever be resolved. However, I want to emphasize that it is just a stage in your life, and you will go through this.
I should also start by saying that there is a reason why you have a girlfriend. Your girlfriend sees the authentic version of you, and no matter whether you see it or not, there is at least one person in your life who sincerely loves you and believes the entire version of you. Of course, she is not the only person. You also have your closest friends, parents, relatives, etc., who are staying with you for a purpose. They all believe that you are genuinely a great person to stay close with. No one stays with a person who is disturbing others. You are already a respective member of their communities and nearby.
Talking to your closest people around you is the first step to feeling better about yourself. They are the closest people in your life for a reason, and I am sure they are happy to answer your self-esteem questions with pride. After a deep conversation with someone nearby, I am sure that you will at least find a direction on how to think about yourself positively. Thinking positively and confidently about yourself is the key to improving self-esteem, and your peers have the supporting keys to opening those doors.
Every time you look at yourself in the mirror, try complimenting at least one thing about yourself and start your day. Instead of feeling terrible every time, allow yourself to find positive facts about yourself. How you wear your clothes and talk about yourself shouldn’t define who you are as a person. In fact, probably there is nothing wrong with the mirror, nor your clothes. It is all about how you approach understanding yourself in those perspectives. One step at a time, you will finally see it, just like everyone else does in you.
Wishing you the best of luck.