You must have broken this person's heart in another lifetime, because they will try their hardest not to let that happen again in this life -- on a subconscious level, of course. They will try to control you if you get romantically involved with them, weighing in on how you conduct yourself, which friends you see, what clothes you wear and more. Their rigid rules for how you must behave if you're to be involved with them will wear on you, but you might find it hard to leave them; after all, those past-life ties run strong between you, and you may even feel a sense of obligation toward your sweetie.
They will behave as if they have some sort of natural authority over you, which may also be left over from a past life; they could have been your parent or your mentor, and they were in charge of making sure that your behavior was up to their standards. But that is an inappropriate dynamic for this life, as you'll be lovers, not student and teacher. If you're to stay with them long-term you'll have to find a way to communicate to them that you're in charge of yourself, not them. Part of this process may involve being compassionate regarding their insecurities within the relationship, and working to establish a firm foundation for your feelings for each other that doesn't rely on control and submission. If your romantic advances are rejected, try not to take it personally; remember that your lover is under the sway of some powerful insecurities that have their roots in another time and place. You may have to think hard about whether working through this karma is worth the benefits of staying in the relationship.