I’m sorry for my email is very short, but I have to go now.
I’m sorry that my email is very short, but I have to go now.
Use sorry for before a noun or an -ing form: ‘I’m sorry for the inconvenience.’ ‘I’m sorry for not writing sooner.’
Use sorry that before a subject and verb: ‘I’m sorry that you couldn’t contact me sooner.’
Do not use for before a complete clause such as ‘my email is very short’.