Throughout the course of The Vampire Diaries and The Originals Rebekah searched for an eternal love. Although she did finally find what she was looking for, she spent centuries trying to deal with the trust issues she has that ruin almost every relationship she enters. On top of this, she had to deal with her overprotective brothers constantly judging her choice of partners.
With that being said, Rebekah, who was one of the best characters in The Originals, had some good and some really bad relationships throughout her life, all of which changed her in different ways and made her reconsider what she wanted in a partner. Though her time in the franchise ended with The Originals, it's important to go back and remember how she finally got lucky with her happily ever after.
Alexander
Alexander was a member of the Brotherhood of the Five during the 12th century. The Brotherhood was a superpowered group of vampire hunters, who, with each kill, unlocked a different part of the tattooed map on their bodies which lead to the cure to vampirism. When Rebekah first met Alexander, she thought they were destined to be together since he was the first man to take a romantic interest in her.
Being her first love meant that his betrayal was that much more painful. When he attempted to stab Rebekah and kill her, she retaliated and killed the only man she had ever loved. Not only did this make her paranoid and insecure in her future relationships, but the rage that she felt was greater than her love ever was for him. In a way, Alexander made Rebekah the vengeful woman she is in the series.
Stefan
Stefan and Rebekah are not a good match for many reasons. They fell in love when Stefan, who did some of the worst things with his humanity off, was on an endless ripper binge during the 20s. Their addictions to blood, love, and sex are the only reasons that they wanted to be together. Well, that and the fact that this was one of the few relationships that Klaus actually approved of because he loved Stefan like a brother and wanted to see him happy.
Though this couple certainly had fun together before Klaus staked Rebekah and compelled Stefan to forget the two of them, when they met again after Klaus made Stefan become his servant, Stefan became hostile towards his former lover. She still obviously loved him, but he was stuck on his new obsession, Elena Gilbert. This, too, made Rebekah even more vengeful and closed off from love.
Matt
Matt and Rebekah weren't terrible for each other, proving to challenge each other in a healthy way. Because Matt was human, Rebekah could compel him and make sure that he didn't plan to hurt her and Matt got to date one of the most dangerous vampires in the world. His boy-next-door archetype also softened Rebekah because he was genuinely sweet toward her.
Although he did end up tricking her at one point to help out Elena and his friends, Rebekah didn't try to kill him as she did to Alexander because she knew he was just thinking about his friends the same way she always thought about protecting her family. Not to mention, being with a human again made her realize how much she missed being a mortal being who could grow old with her lover.
Damon
Damon and Rebekah always get the bad rap when it comes to being judged against their siblings. They are both impulsive, reckless, and sometimes just a little naïve when it comes to love. Although they didn't engage in a fully committed relationship, they got together because they understood each other in ways that other people just didn't. They both believed they could never have the love they wanted so they settled for each other to distract themselves from that reality.
While her brother didn't exactly approve of this relationship and Damon, who wasn't always a great boyfriend, still betrayed Rebekah on countless occasions, they never held long-standing animosity against one another. Whenever they started to fight they usually ended up turning that anger into heated sex that made them both forget what they were mad about. Damon taught Rebekah to just enjoy the feelings of love without worrying about the repercussions.
Marcel
Marcel and Rebekah, one of the best ships in The Originals, were meant to be and they gave each other exactly what they wanted and needed in a partner. They both cared and protected each other with all their hearts. Not to mention, Rebekah watched Marcel grow up to be the man that she loved. The only problem ended up being Klaus' anger when he found out about them as Marcel was his best friend and confidant. Klaus felt betrayed by how much they loved each other.
This being said, in time they finally got to have the happily ever after they wanted and live a human existence thanks to the vampire cure, which was all that Rebekah wanted for her entire life. Together, they learned that true love waits until the right moment and will last longer than any natural life.