You said a month into it, you start to emotionally check out. But how long are you actually casually dating them? Hopefully you aren't jumping into relationships 1 month in, because that will tend to lead to relationship burnout or losing interest. You should go on fun, casual, light dates for like 3 months before actually being in a relationship.
That way you can gauge if the person's personality really jives with yours.
So take more time to get to know them. And definitely keep the physical aspect down to a minimum. I know lots of guys who lose interest immediately if it gets physical too fast. You might be one of them.

Let the girl know you want to take it slow and not jump into anything. That way you won't lead her on, and she won't freak out when you stop texting or calling as frequently. Have open communication and let the other person know where you stand and how you're feeling.

You could be a commitment phobe, or maybe you just haven't found someone you really like yet. I guess only you can answer that question.