So for my upcoming D&D campaign I was planning to play a Jekyll and Hyde type of character. I want the DM to just look at me whenever and tell me to roll for whatever stat involves willpower to switch personality or not. So which stat involves willpower?
Unless it's an "evil campaign", don't.
Simply put, unless the campaign is specifically designed to revolve around playing evil characters, and everyone else is on board with this, don't. The identity of "Hyde", in the original novel, was simply an excuse Jekyll used to act on his desires to do evil things. Unless everyone is planning on playing an evil campaign, a character that acts on impulse to indulge his desires is very disruptive.
That said, the stat that represents willpower in DnD 5e is Wisdom, which is why a lot of mind-altering magic allow for Wisdom saves. On page 237 of the Dungeon Master's Guide, it says that Wisdom checks are used for "perceptiveness and willpower", and on the next page, it says that Wisdom saves are used for "effects that charm, frighten, or otherwise assault your willpower". However, there are no mechanics for rolling your Wisdom to change between split personalities.