Sometimes I just need to sleep in my own bed because these beds are so small how can I get my girlfriend to understand?

Be clear about it. In addition, u may sleep jointly on a matress or buy a bigger bed.
Be up front. Suggest you share a bed for your intimate time with each other and when it's time to sleep, both can move to a separate bed. Let your girlfriend know that you are a much better partner when you can get a good nights sleep. She may not know how you feel if you don't tell her. Telling her may bring out things about her feelings you may not know. That may change some of your thinking as well.