Can I use the phrase 真是乱七八糟！in these ways?

Hi, Beth!
Yes, it means 'what a mess, at sixes and sevens, out of order'. Generally, however, 乱七八糟 is used when some place is awfully untidy. e.g.Nigel的房间乱七八糟./Jen的抽屉乱七八糟.
You may also use it as an adj with的,eg.乱七八糟的抽屉,乱七八糟的仓库
You may use it this way: sth被sb搞得乱七八糟, meaning sb create the mess. e.g.这是一份非常简单的合同，但是却被Richard搞得乱七八糟./房间被小孩子搞得乱七八糟.
You may also say这是什么乱七八糟的! to show you're very unsatisfied. It is offensive.
e.g.A student had asked his teacher a question, but other than explaining the question well, the teacher kept blathering. The student mumbled这是什么乱七八糟的! And, if your boss is not satisfied with your work, he could also use such language. :)

As for the sentences you gave, they are more about a state of mind, not feeling well about sth, other than the physically untidy. You could use一团糟or糟糕. Simple but effective. Such four word idioms in Chinese as乱七八糟can add colour to your sentence when it is used properly, otherwise, disaster. :)
我的生活一团糟./我的心情一团糟.
我的心情很糟糕./我的心情糟糕透了.透means thoroughly.