I think you need to think rationally here for a moment. Your boyfriend who cheated on you with your younger sister's friend called you ugly and you're taking it to heart.
For one, just as a tip, looks are not the be-all and end-all. I know it's very difficult to avoid being obsessed with them as a young woman in our society, but they really are insignificant. Try your best to feel comfortable with yourself. If changing how you look in terms of hair/makeup/clothing is something that interests you, then by all means do it, but do it for fun (like playing dress-up like you did as a child), don't do it because you feel inadequate.
Secondly, personality is the most important thing about any person. I know this gets bandied around a lot and you're probably thinking 'yeah, yeah', but it's true. Who would you rather spend an evening with? A person who makes you laugh until you cry or a statue?
I understand that you're probably very hurt by this whole situation, but try your best to forget it and remember that it wasn't your fault.