All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
You were the reason for your sorrow whereas you were capable of avoiding it. This is due to you doing a matter that is religiously prohibited as you deceived your husband and pretended that you were another woman who was addressing him. If he was sinful in this, you were the one who caused it; so you are sinful for this as well, in addition to being sinful due to that deceit. Moreover, you spied on him and spying is forbidden. Therefore, you are obliged to repent to Allaah. For more information about how to repent, refer to Fataawa 86527 and 87903.
On the other hand, your husband’s participation in a matrimonial site may not mean that he will really get married. Nonetheless, if we presume that he wants to marry and he is able to be just between you and his second wife, then marrying more than one wife is a matter that is lawful in Islam. For more benefit on the wisdom and evidence of polygyny, please refer to Fataawa 81469, 82154 and 86818.
Therefore, we advise you not to preoccupy yourself too much with this matter so that you would not spoil your marital life. On the contrary, you should beautify yourself for your husband and take care of him so that he would not think of marrying a second wife.
Finally, you should know that it is forbidden to ask for divorce except for a necessity. The Prophet said: “Any woman who asks her husband to divorce her, without a religiously accepted reason, the smell of Paradise is forbidden for her.” [Ahmad]
Allaah Knows best.