Can I marry a person I won't be able to marry in first place if I make du'a?

i am in love with a guy . i went to a person as i was having a problem . he told me to forget the person as you wont be able to marry him ...he is not in your qadr alllah tallah says dua changes qadr ..so if make dua to get marrying that particular person with full faith and belief ... is it permissible ? and how can someone tell that a particular thing is not in your qadr when allah has promised that dua can change qadr