What Causes Thinning Of Hair?

Hello,I am female,27 years of age,offlate experiencing thinning hair.I attempted to grow them long for a year and a half,now they waist lenght..they did grow,but the volume is reduced greatly.My work involves stress which cant be aoided.I have tried various oils but no use.Anything that would really be helpful?And how often should i TRIM MY HAIR