A civil divorce of a valid marriage is not even a sin on your part if a) it was not your choice and was not brought about by your own infidelity or irresponsiblity to your duty as a spouse on your part or b) if you chose it, but with serious reasons having to do with safety or grave matters of well-being, the interests of justice, or the legal protection of your marital assets from an irresponsible spouse. For instance, it is not a sin for a person with a spouse who is a compulsive gambler to obtain a civil divorce in order to protect the couple’s financial future. If the gambling spouse squanders his or her part of the marital assets, the other spouse is also not bound to let them squander the other half, too. The responsible spouse is allowed to have justice and to live in peace.
Even if the civil divorce was due to sin on your part, that is something that you can confess and be absolved from, provided you are not refusing means that are realistically within your reach to mend the marriage. You are still married, but you aren’t required to reconcile with a spouse who will now have nothing to do with you. If you confess your mortal sins, you will be in a state of grace and may receive Holy Communion.
What you may not do is consider yourself free to marry or to act as if you were free to marry unless and until the first marriage is investigated and found to have been invalid. Until your marriage is declared null, your spouse dies, or the two of you reconcile, you have to act like a person who has taken a vow of celibacy. You have to be faithful insofar as it is within your power to be faithful, which is to say you have to refrain from having outside romantic relationships with anyone not your spouse.