Four-part character arc vs. three-act plot structure

I've always struggled with the three-act plot structure. Where to draw the lines between acts isn't always clear to me; the so-called turning points can too often seem too numerous and I'm not sure which one counts as "the important one." (It's a turning point for Frodo when he flees Bag End for Bree, but also when he volunteers in Rivendell to take the Ring to Mordor, but also when he leaves the Fellowship with Sam, but also...wait, is this Act Two yet?) SO, I was e...