Fellas, this one is for you. But ladies, you may find this useful information as well. Here's how to get a little space when you are in a relationship.
Tell your significant other you need to talk to her. Once you have her attention, let her know in very delicate terms that you need some alone time. Even saying "I need alone time," will work.
Step2
Make the conversation all about your needs. Don't bring up anything your significant other has done that makes you need time to yourself (even if he's the reason). You can even go so far as to assure him that it's not his fault.
Step3
Give her a reasonable time frame to expect you to be back and ready to talk again.
Step4
Understand that your significant other will have questions as to why you want to break away and be patient and tactful when answering. Again, make it about your needs and not what he has done wrong, if anything.
Step5
When you come back from your solitude, be prepared to explain what it was about, and again make it about what is going on for you, not the other person. It can be damaging to a relationship for a person to feel like she caused harm when she didn't.