I'm assuming that neither of the parents has Timid Nature. In that case, keep at what you're doing, eventually you'll get a Timid Eevee. The chances are pretty thin though, (1 in 25, or about 4%), so you'll have to be patient.
Also, if you do get a Male Timid Eevee by any chance, just give it an Everstone and breed with it to get a Female Eevee.
An easier method might be to breed your Female Eevee with another male Pokemon of the Field Egg group which has a Timid nature. Attach an Everstone to this Pokemon. Females have a good chance to pass down Hidden Abilities, so one different Pokemon shouldn't make much of a difference.
Hope I helped!