Once you have established or at least understand the basic technical rudiments, you’re on your way to independence. The next step is to forget everything. You should be able to enjoy listening to what you are creating and not worrying about where your thumb is supposed to be. Your body breathes for you, it’s not something you have to think about because your body’s autonomic nervous system does it for you.
If you think about breathing, suddenly you’re aware that you have to inhale and exhale and it becomes overwhelming. Same goes for the thumb. It’s not natural for your fingers to move in this manner so they must be trained. In the beginning try to play* for an hour a day, it won’t take long to pick this up, but it will take a week or two before you’re thumb begins to think for itself. When you get to section two you will see how difficult this seems initially. Just remember to start slowly and learn to tap your foot on the beat. Depending on the song you can tap every beat, or just the one and the three.
Sigh, then take a deep breath. Forget how frustrated you are, and start again.
*I don’t like to use the term ‘practice.’ If it feels like practice put down the guitar and take a break. This is difficult, but it should always remain fun and challenging.