C.J.
I have three daughters and I became a single mother while pregnant with my third - I can remember how hard it was...
1. Don't feel like a terrible mother
2. You can discipline a 14 month old, they are good little manipulators!!
We think that because they are so little and innocent that they don't understand what they're doing. That's partly right, but it is our responsibility to teach them what is acceptable behavior. As much as it totally sucks to listen to the screaming and stomping, or whatever they use to get us to give in - we have to be stronger. She needs to be removed from the room so she knows it is not acceptable to behave that way and that it is disruptive to the rest of the family. Lovingly tell her you would love to have her with you (cooking dinner, watching tv...) as soon as she can be "nice" with no screaming. At 14 months old, if she is still in her crib, put her there and use the soft, loving tone. She has come to expect your reaction and knows she can out-last you :) Like I remind my girls, if you ignore your sister, she'll stop pestering you - your reaction is giving her what she wants.
They are smart little ones!! I found that if my tone was not angry and I stood my ground, they responded better and the tantrums were shorter and farther between.
I know every child is different, I had to tweak it a bit for each of my girls. Plan an outing - to the park or wherever - and remind her that if she has a tantrum it will be time to leave...it is the hardest thing to leave when it happens but she will finally figure it out :) I have left a nearly-full grocery cart in the store more than once and I have turned around many times before ever reaching the park, and it would break my heart but they came to know that mommy meant what she said!
She is testing her boundaries and needs you to help her learn when to stop.