Your question is very good. We can find many discussions in Jewish law and ethics about adultery, deceit and betrayed trust but not about cybersex addiction. At first glance, it might seem that cybersex is not as problematic a form of infidelity as real life adultery. However, this can be misleading. The Internet's ubiquitous freedom can propel the user into a fantasy world. Here virtual personas can realize thwarted desires and fulfill poignant needs. The easy accessibility of the World Wide Web offers charmingly deceptive opportunities for camouflaged intimacy. Cybersex offers immediate stimulation and instant gratification. But it tends to objectify the participants, reducing them to body parts and pornographic pictures while maintaining the secret thrill of an incognito façade. Does your spouse view this as a "harmless pastime"? Or perhaps deep inside he/she would really prefer a healthier alternative? If the latter, then perhaps you can find an opening to gently guide your marital partner to a systematic recovery plan.
In coping with "betrayed trust" in a 20 year marriage, the question is how extensive is the damage. Is the relationship irrevocably torn? If not, then the preferred choice in Jewish law is to try and restore marital harmony, "shalom bayit". Rabbinical courts will usually encourage the couple to search for more effective ways of communicating, sharing and rapprochement. If that is the direction you choose, your empathetic understanding of the motivating factors leading to your partner's "addiction" become a crucial component.
A focal point for halachic discussions on addictions is the Maimonidean description of free will in Mishneh Torah, Laws of Teshuva (Return/Penitence), chs. 5-6, where he explains the gradual loss of free choice in addictive behavior. However, another more optimistic psychological theme has also permeated halachic thinking. It is based on the statement of the Talmudic sage Resh Lakish (himself a former gladiator) in tractate Yoma 86b, that even intentional iniquities can be transformed into merits. This radical concept of "teshuva" was developed by R. Hasdai Crescas (1340-1411), chief Rabbi of Spain responding to the massive wave of conversions to Christianity in 1391. In the 20th century it was propounded by Rav Avraham Yitzchak Hacohen Kook (1865-1935), the first Chief Rabbi of Palestine, in his book "Lights of Return". R. Kook's underlying kabbalistic purpose is to discover the spark of holiness embedded in the sin, and transform it into a force for goodness and strength. In your example of cybersex, the motivating spark is often an essential human yearning for intimacy. The redemptive power of "teshuva" can channel this Eros to a sanctified purity. Then your marital relationship will thrive.
The bottom line: Is this particular case of cybersex preoccupation irreparable? Or is there a chance to renew the romantic spark that initially brought you together? Perhaps it is still possible to transform the iniquity of cybersex into a catalyst for rejuvenating marital joy?