It sounds like you get involved with girls that have the same selfish problems your mother has. I am wondering if your mother is a Narcissist?

Also, you are only 21, that is still very young and so is that age group of girls. Don't form all your emotional decisions and rationalities right now, you are definitely going to grow up a lot the next 4 years. The executive decision making part of your brain isn't fully formed yet, that doesn't happen until around age 25. So, don't beat yourself up if your making not so good choices about girls right now, most struggle with making the right choices at this age, both male and female.