You can open Settings on your iPad and then go to Mail. Any settings which can be modified will be found here. You would want to look for a side pane view or reading view setting. In my research I did not locate an option to change the reading view, so the default view may be the only option available.
Do you have an Apple Store near you? If you take it in there, the staff can walk you through changing this if possible. You can also reach out to support by chat or phone: https://getsupport.apple.com/
Another solution: Install and configure a 3rd party mail app for your email account. There are other mail apps available through the Apple app store. These other mail apps will offer a different look and feel than the iOS Mail app, which may work better for you.
Also: You are welcome to post anywhere in the forum, so the previous location was fine! However since I responded to this thread I removed the other to prevent duplicate responses. Reply directly to this thread here for more help!