A couple suggestions to explore.
First, this will not apply to most people but on the offchance it might apply to you I will mention it, if you find it useful to read about this sort of thing, and find mental imagery useful for correcting your position, check out Mary Wanless's books. Some people find her quite helpful for correcting issues involving being all skewed around or lopsided in the saddle.
Second, get someone to watch you ride. They do NOT need to know the first darned thing about horses
they just need to have eyeballs and a voice. First, have them look at you from the front right after you've mounted, with your feet OUT of the stirrups... have them see whether the stirrup irons are exactly even with each other. Then (don't adjust them!) go warm up for ten minutes or whatever, then have them look at you from the front again. What do the stirrup irons look like now, are they still even? These two pieces of information will give you an idea of whether misadjustment of equipment may be contributing to your problem. (Note that a few people actually *need* their stirrups to be slightly different heights, so it is important to look both at the before and during-ride positions before making any changes)
Third, get that person to watch you WHILE you are riding. Even if they do not know squat about horses or horsemanship, they can tell you simple things like the following:
-When you trot a straight line towards them, are your hips level with each other, is one hip going out to the side, are your shoulders level with each other?
-When you trot a circle around them in each direction, is the axis of your hips and shoulders lined up with the radius of the circle (ie. so you are neither turning your back very slightly to the person in the center as you go around, nor turning your front very slightly to them as you go around) and (HERE IS THE IMPORTANT PART) is it identical in both directions? [I am not proposing to debate the different schools of thought regarding the exact angulation of your shoulder and hip axes on a circle, the real big point here is that it needs to be the SAME.]
I will bet you dollars to donuts that if you have someone watch you they will find useful things to work on in these latter points (one fairly common reason to have a tendency to always fall back onto one diagonal is because you are slumping or twisting in the saddle, as a habit not as an anatomical necessity, and this can be fixed to a considerable degree just by being aware of it and working on it).
And of course whenever you GET access to an instructor that would be good too
but you can do a lot on your own in the meantime.
Oh... and VIDEO, get someone to VIDEO you riding and you may be able to see *yourself* what needs to change. Good video, not itsy-speck-in-the-distance or close-up-of-riders-waist-jiggling-around-in-the-pic. A tripod or a table to rest the camera on is real useful. Have your groundperson do this when they're watching you
Good luck, have fun,
Pat