For context, neither I (18M) and my girlfriend (18F) had ever had sex before each other. We've had sex 3 times, and it's usually fine, but she doesn't like to get on top anymore because it hurts her (I'm 'too deep'). I'm above average in size, but only by about half an inch, so I wouldnt think this should matter. Is this normal, and is there anything we could do to mitigate this?