You have gained powerful magic through a magical experience or person. Maybe one of your parents was a powerful mage. Perhaps you had a powerful spell cast on you as a baby. Or maybe you where born in a area of high magic. However you gained your magic, it now suffuses you. Your magic is more expanded than a normal sorcerer, but you are not quite a wizard. Sorry, but I am not good at writing these things.

Mystical spells

Starting at 1st level, you gain a special ability. Choose 2 spells from the wizard spell list, these are your mystic arcana spells. You gain 2 more of these spells whenever you gain a level in this class. When you finish a long rest you can choose any number of you spells known and swap them with an equal amount of mystic arcana spells. When you level up you can choose one of your mystic arcana to replace with another spell on the wizard spell list.

So if you knew the spells shield and magic missile, and your mystic arcana spells where mage armor and grease, then when you finish a long rest you could instead know grease and mage armor for that day, and shield and magic missile would become mystic arcana.

Now, even though you can swap spells, your spells known and mystic arcana are 2 different things. So if you leveled up and one of your spells known was in you mystic arcana, you can still only change out that spell for a sorcerer spell.

Also, as a bonus action you can swap one of your spells known with one of your mystic arcana spells. You regain use of this when you finish a long rest.

(I do now know how to do this on dndbeyond, so you will just have to write it down)

Innate magic

Starting at 1st level you can cast detect magic a number of times per day equal to your charisma modifier and regain all uses on a long rest.

Advanced mind

Starting at 6th level you gain the following benefits. You can cast any spell this is in your mystic arcana even if it is not one of your spells known by spending 1 sorcery point. Once you do this it doubles in cost. So the first time it cost 1, the second time it costs 2, the third time it costs 4, and so on. It resets back to one when you finish a long rest.

Also, you gain resistance to force damage.

Magical restance

Starting at 14th level you gain protection from magic and learn how to use lingering magic to your advantage. You have advantage on saving throws against magical effects.

Also, as long as you have cast a spell on your last turn, you can add your charisma modifier to one damage roll for all spells you cast on your turn.

Masterful Sorcery

Starting at 18th level you become a master of magic. When you convert sorcery points into spell slots, it only costs a number of sorcery points equal to the spells level.

Additionally, you can now apply 2 metmagics to a single spell. Empowered spell still does not count as your metamagic for a spell. 

Finally, you are immune to any spell of 2nd level or lower that you do not want to be effected by. This effect is so powerful that buff spells of 2nd level or lower does not work against you. So if a paladin had a bless spell on him, he would not gain the 1d4 to his attacks against you, and he would not get his 1d4 to saving throws that come from you. Even if a spell is cast with a higher level spell slot, it still does not effect you if the original level was 2 or less.