I'm not going to tell you you are beautiful, I've got no idea what you look like, but it is obvious that a significant number of people have to be below average when it comes to what is commonly agreed to be beauty at any one time.
How to come to terms with not being one of the lucky ones who are "pretty"?
Well accept it's just luck and doesn't reflect intrinsic value. Remind yourself that beautiful people often do get better service and treatment at the most superficial level, particularly when young, but there is no compelling evidence that they have a better chance of happy, long-term relationships; it brings its own problems. I have an astonishingly beautiful friend who is also very funny and kind; for much of her twenties and thirties she struggled to make friends at work and found people unfriendly. She is so nice and not at all arrogant or pushy so I suspect it was her looks. She changed jobs to an industry that attracts a lot of attractive people and, Lo and behold! - she made friends easily within it. She was also strung along by a man for most of her fertile thirties and had a lot of heartache before finally meeting her lovely husband and having her son, after a loss and with subsequent losses. It wasn't her choice to only have one child and at 41.
Attractiveness and beauty aren't one and the same. Our perception of people's attractiveness is massively influenced by how they interact and present. The man who you assess to be the best looking in the room when you walk in will be eclipsed by the end of the evening by the one with the warm smile, an interest in other people and gentle sense of humour. Even with your low self esteem and current looks, whatever they are, you aren't single, maybe you are with someone who isn't good enough, we can't know, but you can attract.
Finally, although we would all love to be adored in our raw state, both sexes can make a huge difference to their attractiveness through choice of clothes, and in our cases, make-up, - if we choose to. I'm one of those women that covers a pretty staggering range along the plain-pretty spectrum; I'm very short, large breasted and pinkish pale with lips the same colour as the rest of my face, stick me in a oatmeal tracksuit with no make-up and I'm really quite unpleasant looking. The right clothes and a bit of makeup and I'm not at all unpleasant and often perceived as more good looking than I am by people, women as well as men, I think because I'm naturally animated and quite smiley.With the self-esteem, you may have to fake confidence at first, but it becomes a habit and gradually you may grow into it. There seem to be many contributory factors to you low self esteem and none of them have anything to do with your worth, just the inadequacy of your parents and the unkindness of ubiquitous school bullies.
I'm now early 50s and have friends who I've known since school; if I think about my friends, there is no correlation between their looks and their happiness and relationships now. It's just a mix, the lookers are do not have a disproportionate number of the happy relationships.