If you don't want to adopt, then you may still be able to take care of the child informally, much as a babysitter would. You would be a glorified babysitter, but you would never have any legal standing (except under extremely unusual circumstances). And again, what about dad? Would he agree to this long-term arrangement?
So you probably need to do a little research, and start talking to the attorneys in your area using words they understand, and words that indicate to them you also understand what you want, and how to get it. Then, let the attorneys do the paperwork, otherwise you could really screw it up, it's that easy to screw up legal paperwork, if you don't know how to do it.
This is not legal advise. I have merely offered suggestions to focus your thoughts. I strongly recommend you contact a very knowledgeable attorney in your area for further advise.
Good Luck.