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Hi, and welcome to JA!
Have you talked about this with your Rabbi? I'm sure he/she could be very helpful. Regardless, I have provided you with an answer below:
Yes, you can (and should) say Yizkor for a spouse that has passed away, even if you are remarried. I have provided you with two references below that affirm this.
The first reference below states, "The only reason it would not be said is the hurt it might cause the present mate. Being that the yizkor is recited silently, there can be no such fear and the prayer may be recited."
References: http://www.chabad.org/library/article.htm/aid/281635/jewish/Yizkor-Recalling-the-Dead.html (see "For Whom Yizkor Is Recited"), http://www.shemayisrael.co.il/burial/mourning.htm#23 (see #9)
Let me know if that makes sense. Don't forget to press the "Accept" button if this was helpful, since that's the only way I get credit for helping you. Thanks!