ey Kristen, you sound just like me! lol. My dd did the same exact thing, but it started at 13 months. I know they understand time outs, but basically they do the bad behavior so that they get attention, the point of time out isto take that attention away, unfortunately human behavior is to pay more attention to the negative. So when you hold her down to keep her in time out, she is getting what she wants, she may not even know it though. Put her in time out and walk away. DOn't talk to her, look at her or anything. When she gets out of time out, say to someone else in the room, gee I wish dd would stay in time out so she can get up and play with me soon! Or talk to her favorite toy about I I know you miss dd, but she can't play til she sits quietly for a minute. LOL sounds wierd but it works. I have walked over my daugther and called my mom to make it obvious that she would not get attention until after completeing time out.
It takes about a week of constant behavior to form a habit, but if you do it all the time it will work for most parents.
If you have any more questions feel free to contact me directly, good luck1 N.