I've seen a lot of students start in your position. It definitely can be pretty frustrating, but don't get discouraged. Take it one lesson at a time. Your score may not go up immediately (in fact, once you start to use some strategies and implement some new content, sometimes your score can dip just a little bit for a test before it goes up), but it will go up.
Make sure you're interacting with the book you're using - and not just by doing practice problems. If there's something you don't get, make sure that you are taking the time to stop and look it up. It will take time, but it is worth your time to do this because it will save you time later.
Don't try to cram in a ton of new ideas in one sitting. Pick a couple to focus on and practice them. Make sure that you're noting any errors in your
error log (not just that you got it wrong, but WHY you got it wrong too).
And as you take your first practice tests - remember that because the GMAT is adaptive, it's going to feel hard no matter what. If you're getting hard problems, smile - you deserve it! You're doing well.
Just remember: Take it one step at a time, don't overwhelm yourself with too much content at once (practice new skills before you move on instead of trying to speed through a bunch of content!), and remember that this test is designed to be hard. If it wasn't hard, then businesses schools wouldn't care what you got on it!