You cannot be scared of losing someone. No matter how much you get on with her and how much she gets on with you...She will be able to tell you are scared of losing her and will use this to sometimes take advantage of you. This doesnt mean that she doesn't like you as much as you like her. It is just the way people are...You really mustn't be afraid of losing someone.
Also, if you do something by accident and are sorry for it...Just say! There is no need for you to get frustrated with anyone if you have done something wrong, prehaps, yourself...but certainly not your girlfriend. Explain that you did not mean to do it, you just didn't think...It is not selfish unless you thought about how she might get your cold at that very instant, and thought, "Oh well, nevermind" and proceeded to taking a swig from her bottle. Simply apologise and let her know you did it by accident...However don't let "I did it by accident" become a common excuse as then it will become obvious you are just using it to get out of a sticky situation and ultimately lying...