I know what you mean when you say your students get demotivated with the writing feedback they get. It can be very disheartening for a student to see a piece of work covered in corrections after they have spent so much time on it. There are a number of ways around this.
You could use symbols to indicate to students what is wrong. For example: sp for spelling P for punctuation, etc. When marking, you use these symbols in the margins. This indicates to the students that something is wrong without telling them where. They go through that particular sentence to identify the error and correct it themselves. This is more encouraging as students are usually able to correct themselves. It also gets them into the habit of reviewing their work for errors.
You could also tell students before they start writing that you will only focus on one thing when correcting. This could be use of linkers, punctuation, organisation of paragraphs, spelling; whatever you feel they need to work on. You could even give a different focus to each student depending on their needs. This helps students focus while they are writing and makes marking easier for you.
You should always include a positive comment. For example, "you need to work on tenses but your use of linkers was excellent". Give students pointers on how to improve, one or two will be enough. For example, "think about linking paragraphs and work on punctuation on your next piece of writing".
Finally, after correcting work you should allow time in class for students to review what they have done and consider your feedback. Feedback only works if students spend time reflecting on it.