This is best accomplished by finding a really good therapist and getting a prescription for Paxil, Prosac or some other effective anti-depressant.
Relationships end for lots of reasons.
But one common reason is one person just isn't as committed to the idea of what might turn out to be a life-time commitment. The more serious the relationship becomes, the more scared that person becomes of the relationship. On balance, they would rather live a life of shallow, meaningless relationships than to truly and unconditionally love another person, with all the risks associated therewith. You might get a forwarning of this when the person tells you of all their wild and crazy escapades, driving off alone to Montana on a whim, and tout their independence as their number one virtue. An astute observer can spot the beginning of the end of a relationship with a person who wants to run away.