We were in a similar situation. Had been co-sleeping with DS for 4 months, he's 8 months now, and he was a shocking day sleeper.

The co-sleeping itself wasn't an issue for us, it was the terrible days and constant resettling at bedtime & overnight.

It took me about 2 months to come to terms with going to sleep school. I hated every second I was there, but would be lying if I said things were completely different now. DS sleeps alone in his cot all night, and can be put down awake. He's like a different child.

If you're looking for a quick (1-2 week) solution than sleep school would be a good option.