Dont stress about how the other horses look compared to your horse. It could result in you pulling your horses head in. Just take the show as a learning experience and have fun!
There are plenty of threads on this forum about 'collection' 'on the bit' 'suppleness' etc, have a search for those. But getting your horse round is something you have to TEACH, and it takes time. If your horse isnt ready for the show, maybe you should consider not going and working towards another. Do you have a trainer that can help?
Im not going to give any advice on getting your horse to 'round up', because I presume you are riding western? And I know nothing about it! There are heaps of awesome western riders on this forum though, they should be able to help!
Compete for yourself, not for anyone else.
Doesn't matter if your horse isn't rounding up, or performing as good as the next horse - you and there for yourself, to do your best :)
To get your horse to 'round up' - you need work your horse correctly, forward, impulsion, working from the hindquaters. The horse must be balanced, not rushing, not on the forehand, soft and supple. This can take days/weeks/months/years to achieve. Really depends.