Two books that have suggestions I like:
Atomic Habits: An Easy and Proven Way to Build Good Habits and Break Bad Ones by James Clear
Can’t Hurt Me: Master Your Mind and Defy the Odds by David Goggins — warning: he swears a lot, and it isn’t everyone’s cup of tea. The audiobook has extra podcast-style content. Basically, “there is no hack”. You just have to do it. Toughening the mind is a skill that is developed by doing things that you don’t want to do. As soon as a person accepts that self-improvement is going to be an unpleasant process, and every day is going to be unpleasant, the mind accepts that this is the new normal, and it becomes easier. Much of the rest of what he says is commentary on that, but there are some deep insights buried in it.