I would suggest slowing it down...slow it down to 60bpm if you have to. Use a metrenome or a drum machine to play along with. Also, listen to the pattern over and over again so you can get how it sounds in your head.

Another thing to do is practice just the strumming pattern. Mute all of the strings with your fretting hand and strum the pattern. You'll get a percussive effect that allows you to strictly focus on the rhythm of the strumming pattern. This way, you are focused on one single thing.

Also, don't be afraid to walk away from it and come back. You may have to do this multiple times. It happens to me all of the time when I'm learning something very new to me. If you find it just isn't clicking after some honest effort, move onto something else and come back to it the next day.

Be persistent, but give yourself time...you'll get it!