Keep in mind the strum pattern may change and probably does. The same strum pattern usually is not played throughout the entire song. The key can change as well, but you should be able to detect when it changes. Strum patterns are tricking at times. It may consist of 1/4, 1/8, 1/16 beats all mixed up in one measure and change the next measure. Try concentrating on the rythme only and chords. Play everything as 1/4 beats and block out the lead and riffs until you have have the rythme down. Then listen for the up strokes, down strokes, the pauses, and how many are played or not played in your 1/4 beat measure. Then try adding them to you playing. Don't try singing along until you have it down. You will loose you concentration and timing, because the vocal does not hit on each strum.

It will take time, but I'm sure you will get it.