The honeymoon period is not a defined length of time but for couples who go the distance, it typically lasts from six months to two years.
On the surface your sexual relationship fits broadly within that trajectory but if all was well you wouldn't be using words like "fizzled out" to describe your sex life after 12 months.
Chemistry is always more intense at the beginning of a relationship but the sexual frenzy that defines the early months is ordinarily followed by a deepening intimacy, the foundation of longer-term commitment.
As a couple head into their second and third year they are probably not having sex all the time but are still each other's centre of gravity and there is no sense that a change in sexual frequency undermines the quality of their connection.
You had good sexual chemistry six months ago, so clearly something has changed since then. Moving in with each other is a big transition and increased proximity can affect the dynamic in a sexual relationship.
When you lived apart you had to make an effort to see each other. There was a degree of anticipation before you met and that added a certain piquancy to your sexual life. Now you don't have to plan anything and because you can have sex whenever you want it loses some of its appeal.
Sometimes the decision to live together is motivated by purely practical reasons - someone's lease is up or it is cheaper to share - but that doesn't explain why the intensity faded so quickly. Living together exposes you to the full 360-degree view, and that can reveal aspects of a person that you don't like quite as much.
Untidiness, not flushing the loo, terrible TV habits, cooking five of the six sausages in the packet and leaving the last one to putrefy in the fridge - all the quirky little idiosyncrasies that you don't get to see unless you are living with someone 24-7 are not in themselves deal-breakers, but they can create tiny stress fractures in the romantic bubble.
As you say, you made the decision to cohabit relatively quickly, and I wonder whether your age had something to do with that. You don't mention this so I can't be sure it is relevant, but women in their mid-30s tend to be acutely aware of their biological clocks and if a relationship has potential they hit fast forward because they want children.
If that is the case, you and your partner need to be honest with each other about what you want out of this relationship because unspoken anxiety is the fastest and most effective way to snuff out the sexual spark. One honest conversation may be all it takes to unlock everything, and it is absolutely possible to get things back on track if you are both equally committed to the relationship. Are you?
You don't say anything about your partner or how you feel about him, so I don't know what you want and I suspect you don't either. In that situation, the best idea is often to toss a coin. Heads you stay. Tails you go. Toss the coin, and if it lands on heads and you don't feel an immediate sense of relief, listen to your instincts, find a gentle way to end the relationship and move on.