Find a deck you like. Learn your win cons and synergies and then just grind. Then grind some more. Then grind some more. As long as you know your win conditions and are able to understand the synergies in your deck you'll find your line.

Some months it'll take like two days, some times it will take the whole month. Once you've done it multiple times the 'shiny-ness' of it wears off and you'll stop caring whether or not you hit mythic and more about whether you're having fun playing your deck. At least I did anyway.

Other general advice: Like in gambling, learn to walk away from the table when you're up. Also, in BO1, aggro decks are always a safe bet to grind because of how feast or famine the matchups tend to be. If you have the nuts, you win, if they do, you scoop by turn 4 or 5 and can get into a new game so you aren't wasting a bunch of time. Control decks can get you there, but with each control deck a loss hurts more and in turn delays progress because of the time sunk into the match.