It's pretty common to get wildly different scores on practice exams. It sounds like you're first order of business is to get comfortable with verbal questions. One way to get comfortable is through practicing lots of verbal questions in the official guide.
As for your discomfort with GMATPrep, it's not uncommon for people to struggle with the transition to GMATPrep. The GMATPrep format is not elegant and the font stinks, but it's also a problem of making the transition to real GMAT questions. The only way to get comfortable is exposure. Your practice in the OG will help, and you can also do additional GMATPrep work, including GMATPrep 2.0 (skip the IR if you take the GMAT before June 5th).