We are sorry to hear of your marital situation.
If you are in niddah, your husband must know because of the various halachot involved. In your situation, you are not obligated to divulge your status when tehorah to your husband, nor are you required to continue immersing in the mikveh. However, you could still consider doing so.
In a situation such as this, where a decision has been made to divorce, there is no requirement of relations after mikveh. Nevertheless, by continuing to immerse, you would have the option of resuming contact should you wish to do so, as improbable as this might now seem. We have received questions from women in similar situations for whom reconciliation became a sudden possibility and who regretted not having immersed on time.
If you have not done so already, we urge you to consider counseling. At best, it might save your marriage. At least, it could help you navigate the divorce process. Referrals for religious counselors are available through Nefesh International, www.nefesh.org