If your child has autism and you still want to be in a relationship with your girlfriend, but you just want to live on your own... I dunno man that sounds extremely selfish. You don't just have a "casual" relationship with a woman if you have a freakin child with her. Unless that feeling is 100% mutual you're just going to hurt her and your child.
Do you have any idea how much it would hurt to even hear that from your partner? Say if the tables were turned. You loved her, and she tells you "well you know, I love you but I just can't stand being in the same house with you 24/7, I'd like to live alone and have a casual relationship instead" Pretty sure that wouldn't feel good at all. There's no in-between here. You are either in or out. Unless you both happen to be one of those weirdo swinger couples that enjoys keybowl parties and shit like that.
Do yourself, your daughter, and your partner a favor. Think really long and hard about how you feel about her. If you can't stand the thought of spending the rest of your life with her you clearly made a mistake. Keep in mind all relationships have ups and downs. Couples will always have issues and it's normal to not see eye to eye for many things. I would hope that you at the very least brought up your issues with her first. You two need to talk about it otherwise nothing will change.If you split, It's not the end of the world though. Best to move on and figure out whats best for you, but keep in mind whats best for your child as well.