By the sounds of things, while trying to look this up you've actually found some of the rules from 5th edition Dungeons & Dragons, which is definitely not Pathfinder. In 5e D&D, it is indeed the case that when you score a critical hit, you roll the attack's damage dice twice and apply your flat modifiers only once. However, in Pathfinder, the rule is different:
A critical hit means that you roll your damage more than once, with all your usual bonuses, and add the rolls together. Unless otherwise specified, the threat range for a critical hit on an attack roll is 20, and the multiplier is ×2.
Exception: Precision damage (such as from a rogue’s sneak attack class feature) and additional damage dice from special weapon qualities (such as flaming) are not multiplied when you score a critical hit.
You actually get to multiply pretty much all your damage, not just the dice; the only thing that Pathfinder excludes here is anything typed as precision damage and extra damage dice from special weapon qualities. This means your strength bonus, your power attack bonus, your weapon's enhancement bonus, etc. etc. are all doubled when you critically hit with your greatsword.
So, for the sake of argument, if your normal attack did 2d6+36 damage, a critical hit with a x2 multiplier would do 4d6+72 damage.
(Mythic) Power Attacks
Now let's calculate your power attack damage! (I will ignore the weapon damage and all the other damage bonuses you have calculated, since the power attack is the point of confusion.) Here's all the relevant rules:
You can choose to take a –1 penalty on all melee attack rolls and combat maneuver checks to gain a +2 bonus on all melee damage rolls. This bonus to damage is increased by half (+50%) if you are making an attack with a two-handed weapon [...]
When your base attack bonus reaches +4, and every 4 points thereafter, the penalty increases by –1 and the bonus to damage increases by +2.
For a character with BAB +19, they would normally take a -5 penalty to hit and gain a +10 bonus to damage. When using a two-handed weapon, the bonus increases by 50% to +15.
Power Attack (Mythic)
When you use Power Attack, you gain a +3 bonus on melee damage rolls instead of +2. When your base attack bonus reaches +4 and every 4 points thereafter, the amount of bonus damage increases by +3 instead of +2.
In addition, the bonus damage from this feat is doubled on a critical hit, before it’s multiplied by the weapon’s critical multiplier.
Mythic Power Attack increases the damage bonus from +2 per point of attack penalty to +3, so the BAB +19 character is now taking a -5 attack penalty for +15 bonus damage. When using a two-handed weapon, the bonus increases by 50% (as per the normal rules for power attack) to give a total of +22 bonus damage - the fraction is rounded down. (I'm not sure how you arrived a +24 power attack total in your answer, but I think that is incorrect.)
Now when you score a critical hit with your greatsword, you would normally multiply that - however! Mythic Power Attack has a special clause that when you critically hit, your power attack bonus damage is pre-emptively doubled before being multiplied by the critical. So, in your case, your +22 bonus damage doubles to +44 bonus damage, and then is doubled again by the normal x2 critical multiplier - so the power attack part of your damage on a critical hit is actually +88.
By the time you have Greater Power Attack, you'll have BAB +21, so your power attack penalty will have increased to -6, and your base (mythic) power attack damage bonus will have increased to +18. Normally, using a two-handed weapon would increase that by 50% to +27. But since you will have Greater Power Attack, the 50% increase becomes a 100% increase, and your bonus damage will instead go up to +36. If you then score a critical hit, your bonus damage doubles to +72 due to the special rule of Mythic Power Attack and is then doubled again by the critical to an almighty +144.