Instead of trying to use something else, just drop わたしは altogether.
わたしは げつようびは べんきょうしませんでした。かようびは べんきょうしました。
And it's usually perfectly fine to drop the first わたしは as well. As long as you can infer the subject from the context, you don't have to (or shouldn't) specify the subject explicitly.
げつようびは べんきょうしませんでした。かようびは べんきょうしました。
Japanese is a topic-prominent language, and it's perfectly fine to drop subjects.
Another feature of subject prominent languages is that they usually require a subject. This is particularly true of English, which has to insert the dummy subject “it” if no other subject is apparent (e.g. in “It’s raining.") Really the topic of this sentence is the rain, but English can’t just have a sentence with no subject, so it inserts a redundant one.
Again, topic prominent languages don’t have this problem. Mandarin, Japanese and Korean are all known for frequently dropping the subject from sentences when it’s clear in the context. The subject isn’t the most important item in a sentence, so it’s fine to do without one.