I have a couple of comments.
1. The matter of keeping a rhythm or pattern going is a matter of practice. Are you practicing the rhythm or pattern, or are you trying to play it without practicing it? If the latter, then practice it. That means to dissect it, slow it down, concentrate on that and only that, and then do it until you can do it steadily (albeit slowly) and then speed it up until you can do it quickly and then keep doing it until you can do it without thinking about it. This is not a matter of 1/2 hour. It is the matter of 1/2 hour a day for however long it takes. Some folks who are already adept at learning strum patterns, 1/2 hour may be enough. For folks who are just beginning to learn them, 1/2 hour a day for a month may not be enough. But rest assured that it will come in time.
2. Forget what it's "supposed to sound" like. Play it the way you like. Until you've been playing for three decades, have gone to conservatory, and beat out 1,200 other accomplished guitarists trying to get the single studio position to back up a famous lead singer, what "it's supposed to sound like" doesn't mean squat (at least not to me). I think it's much better to be able to interpret songs to your own style, even if you don't have one yet.
3. Nothing wrong with simple melodies picked out. That's something I can't do yet, so don't run yourself down for that. I'm can get the strumming patterns fairly easily, but not the melodies picked out. We each bring what we have, and have to learn the rest. Learning takes practice and time. You'll get where you want to go if you just relax, put together a plan to get there, and make the effort through relaxed and focused practice.
- Zurf
Granted B chord amnesty by King of the Mutants (Long live the king).
If it comes from the heart and you add a few beers... it'll be awesome! - Mekidsmom
When in doubt ... hats. - B.G. Dude