Dating..But Filing Taxes as Married Filing Jointly...Meaning?

Of the 11 or so states that have "common law marriage" the rules are different in each of them. Generally the parties become married when they live together and do things as if they were married, like file tax returns. Under the cirstances you describe you very well may be legally married to this person. You would be well advised to seek counsel from a divorce attorney ASAP. If in fact you are married via a common law marriage in your state you must also get a divorce to dissolve that marriage just the same as any marriage. You potentially have a number of issues you really need professional help with including any tax issues that may have resulted from his filing joint returns.