Du`a’ and Qadar

As salaamualaikum wa rahmathulaihi wa barakathuhu May Allah bestow his blessings and mercy upon you for guiding those in need.My question relates to Qadr or predestination in Islam. It is a belief that dua can to an extent change and influence a persons predestined path of actions. My question relates to marriage. I?m currently in a relationship with a woman and things aren?t looking too good for us in terms of getting married due to vast cultural differences. We are praying hard that things work out and we do get married. My question is that if we do ask dua and if Allah does will for us to get married then is it in some way gonna affect the way our destinies should have been and cause a big change in out futures for some reason. Also we are asking dua because we are in a relationship which is not allowed in Islam in the first place and if we do get married then will it in any way bring in a bad affect to our marriage if we were predestined to be with someone else.My second question is that if marriages are predestined and being in relationships is wrong then how would Islam explain a couple in a relationship who go on to get married. Would they have found each other and gotten married anyway.Finally if dua can change a persons predestined path of actions then does a person who asks a lot of dua in someway decide his own destiny, maybe not at everything but some major aspects which may have a big impact on his future.If the best thing in pleasing Allah is ending this relationship I would appreciate some advice on how best to do it. Jazak Allah Khair.