It takes two people to make a baby, and both of those people need to be ready. Also you mention that you want a girl. What happens if you don't get a girl? What happens if your fiance doesn't want a baby but you get pregnant anyway? Have you thought out all the possible scenarios if you don't "get what you want"?
As far as your 2 year old "getting lonely" have you tried to make playdates with friends or getting involved in childrens' classes, park programs, or library story time? There are many ways to provide him with social opportunities without making another baby.
Wait til you get married. If your fiance doesn't want a child now and you force the issue he could be resentful and it could cause a major strain in your marriage. It could possibly end up where you don't get married, end in a divorce, or the marriage isn't happy because he feels 'forced'. Think of the impact that could have on not just one child now but two.
You need to have a long talk with your fiance. Having children is not all about what 'you' want but what the 'two of you' want. It's not just your life that changes; his does too.