Hi Jenny,
here's a "trick": Practice swinging your arms to the back, as though you want to jab somebody behind you with your elbows. Exaggerate the motion as a training method, for a couple minutes at a time, then relax your arms. After a while, your arms will find the optimal position and that'll help straighten your back.
You can also use the arm swing to get your stride rate under control. Swing your arms, actually jab them very quickly and "snappy", up and back, your legs will move at the same tempo, your stride will shorten a bit. Again, just do it for a few minutes, as a training tool, your general form will improve over time.
Somethine else: When you're good and warmed up, or maybe when you start to feel a little tired, do some active stretching, while you're still running. My favorite is to stretch my arms up over my head, fingers interlocked, palms skyward. Look up, push your arms to the back, lot's of tension, open up the front side of your body, your shoulder blades willl be pressed together. Hold this for 10 or 20 strides, really push up and back, then let your arms sink to the sides, shoulder height, palms to the front, arms outstretched as far to the sides as you can, and push to the back again. Tension. Hold this for another 10 or 20 strides, and then just relax back into your running form. Feels great and helps over time to keep your upright.
Hope that is all understandable and actually useful!
W