I'm glad other people have had helpful insights and suggestions for you. Few of us are supermodels and none of us walk around with the benefit or airbrushed perfection or professionally photographed lighting. I can look awful in a picture or I can look not too bad. I'm not a glamour girl, so we're talking about ordinary appearance. Try to find your best feature and appreciate it. A plain ring may have a single jewel that shines, and so must you. Try to emphasize your best feature(s), because doing so may create some enjoyment for you. When I was younger I never looked in a mirror and felt uncomfortable doing so. You may be the same, but if you can get past that, you may find doing your best with what you have does feel good.

Looks fade, so other virtues are also good to focus on. If you are young, as you seem to be, you are no doubt capable of looking great, even if you are plain. Try to make the most of it, and get some enjoyment out of it.

Maybe this is not the approach you want, the external one rather than the more meaningful internal one. But since it made a difference even for an anti-fashionista (me), I thought it might be a net positive for you. Don't compare yourself to others, but get used to setting your own standard and comparing to your recent self. Just an idea. I was struck yesterday with awareness of comparing myself to someone I admire greatly, someone I have had only a professional relationship with, but close proximity. I suffered greatly with this over the past week, and it is just fading a little, so I can relate to your pain in that sense.