what happened after the marriage is not considered in an annulment investigation, but the situation that pertained at the time of the marriage. If facts are as stated (and it is not possible in this forum to get all the facts) the marriage was not valid due to lack of form, that is, he was a Catholic who got married outside the Church. If you were to convert, as it stands, nothing need be done. If you ever want to remarry in the future, you would need to submit the facts of your first marriage to your pastor and related documentation. Among other things, proof he was Catholic, your marriage and divorce papers. If facts are as stated, no full-fledged annulment procedure is necessary, just a rather simple paperwork thing, but it still must be done.
When a non-Catholic approaches the pastor and asks to be prepared for reception into the Catholic Church, the first thing that happens is a rather in depth interview to bring out issues such as this. That gives the pastor time to investigate and see what, if any, action needs to be taken.
welcome home
you need not be Catholic to ask for an decision on the validity of your previous marriage to a Catholic. you can approach the nearest Catholic church and ask the pastor to begin the process now. The sooner the better is usually good because it is easier to get the documentation you need.