when she goes down on me its hard for me to stay hard. Is it me or is it just because I haven't experienced this before?
If this is the first time you've had this experience, it could be nervous anxiety on your part. After a few times that will change. If the girl is one you really favor and you want to make a good impression, maybe it was a psychological hang up that kept you from succeeding. Or, if the girl is new to this and isn't very good at blowjobs, then you may have experienced disappointment and you lost your erection. With some practice all that can change.
Being naked, getting a blowjob where there is enough light for you to observe your cock being swallowed should be enough turn-on to keep you hard. If you are young and hung and normally have a good sex drive, you should be able to stay hard when someone goes down on you enthusiastically. I hope you manage to find a partner like that.