Unfortunately I have seen this on numerous occasions, a Non-immigrant visa multiple entry is normally a 1 year visa and obtained in your own country from a Thai consulate before you arrive but the immigration law states you have to leave Thailand every 90 days via the airport or a Cambodian visa run and then you can re-enter immediately as long as your visa is still valid. In your case you have now overstayed your visa for 9 months and my advice is to go to your local Immigration office and point out the mistake and explain this was an oversight and not intentional and in most cases the officer will allow you to pay your overstay and give you a 7 day extension to give you time to exit Thailand on a Cambodian visa run. The reason we see this numerous times is because in most cases when you purchase the visa the rules are not fully explained and most people are still not checking there visa stamp when they come through the airport.