I might be slightly biased because I have a child, but that is really no reason for you to look down your nose at someone. Having a child is hard work and not a lot of people really realise how hard until they are in that situation - being a single parent is even harder. Not a lot of people given younger mums credit, because they're all branded with the same brush.
She might only work in a pub, but she is working to keep a roof over her childs head. You have no idea what her future ambitions are/whether she has any. Don't assume she wants to work there forever.
You also seem to assume a lot about this girl - the child 'apparently' doesn't live with her, she 'apparently' snorts coke etc. Being a law student you should know not to make such assumptions. My son technically lives with my parents, but so do I. I also get the chance to go out most weekends, because my son visits his other gran - I am there for him leaving, I always have my phone on me in case I need to get to him/get home for him, I am always there when he is home etc. Just because someone goes out it does not mean they are dumping their child. My son sees his other family, and I have a day/night to do as I wish, whether it be sit in or go out - I chose to see my friends. That does not make someone an awful parent - again, get some evidence before making/implying such assumptions.
And lastly, the whole looks thing. Did you really have to point out that she was 'rounded'? That doesn't make one iota of difference, and just makes you appear quite shallow and vain.
You gave the thread the title "Shall I become more like the girl my ex left me for?". In most cases someone would be singing their praises and wondering what they themselves had lacked compared to this girl. You are sitting there essentially saying you are better than her, she is nothing, you've no idea why someone would leave you for her etc. You seem to have a high sense of self-worth and importance and disregard anyone who does not have the same looks as you or the same education level.
If I seen someone do that to people, I wouldn't want to be with them either.
And for the record, I have also been cheated on. I also wanted to be the girl they left me for at first. Did I bring those girls down however? No. I sat and tried to make myself be them because I thought they were better than me, not the other way about.
I'm also not a benefit scrounging slag. I'm a fourth year law student, almost graduated, and a part time civil servant - before anyone starts.