It has been bothering me for a while- a year. My husband divorced me and moved onto another woman, but my concern is that I am now in motal sin because of that. IS it a sin to be divorced even if I remain celibate and do not date?
This question is too complex for a simple yes or no answer. being divorced, per se, is not a sin. But the circumstances under your control that led to the divorce may or may not have been sinful.
On the one hand, it is possible for a divorced person to be an innocent victim, whom, if he or she does all that is possible to reconcile with his or her spouse, or remains chaste and does not attempt another marriage without a declaration of nulity, in this case is not guilty of any sin related to the divorce whatsoever.
On the other hand, if a divorced person is also responsible or partially responsible for the breakup of the marriage by reasons of his or her own sinful actions or omissions, then that person is guilty of those sins, and possibly for the divorce as well, with the gravity of that guilt depending on those particular circumstances.
If you are morally certain that you did all you could have and should have done to save your marriage, then you are guilty of no sin relating to your divorce at all. If you know you failed to do so in some ways, then you may share in some venial guilt regarding your divorce. But if by your own free choices and actions you damaged your marriage or failed to build it up when you should have, then it is possible that you may be seriously or mortally guilty.
I won’t ask you for details to try to evaluate your innocence or guilt myself because that isn’t my business, but I would suggest you have a talk with your priest about this if you are unsure.