My girlfriend is always dry during sex and hard for me to easily penetrate, what could be the reason for that and how can it be stopped?

Vaginal lubrication: There are a few different reasons why this may be happening. The most common reason this happens in a younger woman may be because she may not be adequately aroused to have sex. Taking more time with foreplay and speaking to her about this may help there are also hormonal reasons for this. Using a good lubricant (such as k-y or astroglide) may help.

Dyspareunia: Typically, women will become adequately lubricated w/adequate sexual excitement. However, anxiety can prevent lubrication. But if that isn't the case, then water-based lubricants can be used. Lubricated condoms provide additional benefit by decreasing risk of sexually transmitted diseases/infection when used each & every time as directed.