Once you eliminate any tack and health issues, what I see the majority of time is crooked riders making it difficult for the horse to carry the rider straight. IOW, the rider causes the crookedness from their own crookedness. This is more often the case with lighter breeds, like Arabians. Most riders are really too big for Arabians. Therefore the issues of the riders crookedness has a proportionately larger effect on the horses crookedness than a smaller rider on a larger horse. It's simple physics. Luckily, Arabians can carry more weight than other breeds of the same "size" however, in order to stay out of the way of your horses movement, you have a larger responsibility to work on your position in order to make it easier for your horse.
Or get a bigger horse.
If you can learn to truly ride dressage on an Arabian, you can ride, truly ride, dressage on anything!
Chris (and everyone else) is spot on! I have been through all of the above over the course of nearly two years (took THAT long because, like lots of others unemployment/financial ruin put a damper on luxuries)! LOL My mare started her serious training for dressage at five/six years old and when she started resisting, we visited the tooth fairy and found wolf teeth. She improved, muscled up and her saddle didn't fit. Had reflocked and fitted. STILL didn't fit. Searched for wide saddle, found used Duett and bought it. Every girth I tried rubbed her raw. FINALLY found affordable contoured dressage girth and her bit/bridle began chafing! GAH! She takes a pony headstall, horse noseband, cob browband and must have a bit with small rings because her face widens SO quickly, they'll rub her face (bit guards don't work).
Every time we achieved more self carriage, the change in how she carried herself caused a tack-fit problem! HANG IN THERE! It will get better (: Most helpful is when I do a lot of flexions at the walk, emphasizing that I want my mare to fill the outside of her body into my outside leg as she bends around my inside leg. That way, I can give her some slack on the inside rein and she is rewarded for the bend. As you advance, you can work on shoulder in to straight and back to shoulder in up the long side. Jane Savoie has a great vid on Youtube you might want to watch.