Don't swing first. Corrin is extremely good at whiff punishing herself. Even something like landing with nair/fair next to her, usually safe vs most characters, can be reactively punished by Corrin with Instant Pin. Same with rising fair/nair, which can be punished by rising fair/nair from the other Corrin.
Corrin can also exploit the fact that her limbs extend/stick out during some of her moves (like DL, fsmash, dsmash, etc) and get an Instant Pin. This isn't usually an issue vs most characters but DL being very quick and long range makes it dangerous to throw out these moves in the zone where Corrin is supposed to be comfortable.
It's also a good idea to fade back after a string/combo is over to avoid getting naired or daired for overextending.
I've only played the ditto a few times on FG, and it's probably one of the least enjoyable dittos (compared to the likes of Falcon dittos). Patience is key, and keep in mind that losing the ditto doesn't mean that your Corrin is necessarily worse; Corrin vs Corrin just happens to be one matchup among 57 others that requires its fair share of MU knowledge. That said, the ditto is also a good way to learn where your character falls short and struggles, and Corrin happens to be quite good at destroying herself.