These sentences are not complete sentences. reading the book => I was reading the book. working on task 123 => I was working on task 123 The Past Continous is wrong for this situation. The Simple Past is correct for a finished time period such as "yesterday." Yesterday, I read the book. I worked on task 123. I highly recommend Grammar in Use by Murphy (Cambridge). It has two-page lessons which consist of a page of examples and a page of exercises. The book has a self-test which helps you identify the grammar points that you are weak in and which lessons to do. Some editions have the exercise answers which is helpful if you are studying by yourself.

The easiest answer for this is to tell you to use the simple past. This tense is for actions in the past that have been completed. For example: I worked on task 123. This is the best tense to use for what you are describing; activities that you completed and are now writing about them. Just be careful with some of the verbs that you have chosen because the tense implies that you finished the activity. For example, if you say, "I read the book", I could understand that you finished reading it, you completed the book. Better to say, "I read chapter 2", "I did some reading in the book", "I read some of the book", etc. The same goes with learning. If you say, "I learned the language", it would be understood that you completed your learning of the language. That is probably not the case! It might be better to use a verb like "studied" or mention something specific that you learned. "I learned about the simple past today", for example, or "I studied phrasal verbs". It's great that you're keeping a journal, it's a really good idea!