"Time travel can be hazardous to yourself"

Own Worst Enemy

James Keaton invents a time machine and goes back seven years to try to thwart the plans of his rival, Frederic Nichols. Meanwhile, Keaton's neglected girlfriend, Leila Locke, finds the time machine and uses it to convince her younger self to not get involved with Keaton in the first place. While in the past, however, she falls in love with the younger Keaton all over again.