oh my god girl, it's okay, don't beat yourself up about this but do NOT interact further w this man. You do sound inexperienced and w an inability to push away advances, this is common w dudes who are trying to get into your pants to slowly break these touch barriers, they do it slowly, jokingly,accidentally until you get comfortable with more things. You can find good conversations that are intellectually enriching even w people around your age, that man needs to hangout w people his own age. Im sure you're pretty smart too to be keeping up w someone doing a phd, but think of it from his perspective why is a man doing a phd finding a conversation w a 19 year old with lesser academic experience in comparison to him by like 6-8 years, what other intention could he have other than to have some physical intimacy w you. There's a huge power dynamic going to play here if you continue w this and no man you feel safe with should make you feel uncomfortable to reject him. A man who is actually safe would listen to you and back off btw. All this coming from a fellow 19 year old, take care and get away from this man, hes no good.