I want to get an abortion without telling my fiance
Why would you want to avoid telling your fiance?
I don't think I can handle having a baby but I know he would want to keep it.
I understand you feeling like you aren't ready to have a child but who really is? You should talk to your significant other about this instead of just avoiding him altogether. He is part of the decision too as he is the child's father and you might forever damage your relationship if he finds out later.
I don't want him to be upset with me.
This will only get worse if you don't tell him what's going on. Try sitting him down and talking to him about it.