You've spent your whole life training and practicing monogamy. You are going to have some painful adjustments to make as far as your emotional reflexes go. It takes time and experience with these new situations to see and really believe/feel secure.
Being very open and communicative with your husband and trying very hard to not have knee jerk reactions and obsess is going to be the key. It's like when you first started your relationship so many years ago, this situation is going to require emotional and social skills that you may have not really ever needed. Just don't make hasty decisions, move slowly, and keep the lines of communication open.