Try compose email by pressing New Email >
Options >
There's a "FROM" Button >
You should see something here >
Otherwise, setup your Sent From email by selecting the dropdown button and choose"Other Email Address"

The best step is to do a backup of your emails and contacts or you can copy the pst file to an external drive.

Then, delete your account in the outlook and then setup all over again. After that make sure that you import back the pst file that you backed up.