In my experience, a new habit needs 30 days to bed in.
You still have to work at it after that, but it gets easier
If you're still finding it tough going, maybe don't try something else quite yet. It is better to be successful with one thing than to fail at two!
There are various research about this. Some say it takes 21 days to form a habit, while others say it takes 66.
If your habit is a single habit, then I don't see any problem with adding a new habit straight away.
For example, if you want to eat fruits for breakfast as a habit, then you can add any other habit for other parts of the day, like reading a book 30 minutes before bedtime or something else.
But if your habit is a routine, such as waking up at this time, reading a book, going to work early, focusing on the important tasks, go home and cook a healthy meal etc, then yes, you want to focus on this routine until you've managed to consistently do it (probably more than 21 or 66 days).