It sounds to me as if you're saying you can follow tab but don't know where to begin when just playing yourself... if I'm right, then what you need to do is practice rhythm more... start by playing 4 downstrokes of a chord, then try 'down-up-down-up-down-up-down-up'. Once you can do that, you'll be amazed what variations you can come up with just by missing strums here and there intentionally.
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