Charles wanted to marry Camilla in the first place, but as I understand it he was told that she wasn't suitable (by Prince Philip I think), because she'd had previous boyfriends and Charles' bride should be a virgin. Charles was then sent off somewhere (Navy maybe or on a royal tour) and by the time he returned Camilla had married Andrew Parker-Bowles.
Due to the virgin thing Charles then needed to find someone who fit the bill and was also a member of the aristocracy - enter stage left Diana Spencer. She fit the bill, Charles was pressured to make a snap decision and the pressure was also on for him to marry and produce heirs and so he married Diana. Yes, she was used. She was young and aristocratic and was in the right place at the right time.
The relationship was doomed from the start, of course. Diana was ill-prepared and ill-suited for the role she was thrust into. She and Charles had nothing in common. And he had already met the woman he wanted to marry and although I think he tried to make a go of things with Diana, they were just too different and so he drifted back to Camilla, while Diana looked for someone more her own age and with whom she had stuff in common.
As for who I support - Diana has been dead for 24 years, which is an utter tragedy - but IMO Charles and Camilla should've been allowed to marry in the first place. Their relationship has stood the test of time and they clearly are the love of each other's lives. Camilla has had a huge amount of criticism, but she has never spoken out in her own defence, always maintaining a dignified silence, a good quality in a future queen consort.