You may not have cut the glue properly. Use a single edge razor blade, and cut between the glass and the glue to separate the glass from the glue. Do this on both outside and inside. When the frame is ready you will feel it start to give. If it does not feel that way, cut between the glass and the glue again.
If the frame breaks, I have two sources for you. One is glasscages.com, then find and pick "Tank Frames" along the left side.
The other is cciaquarium.com
http://cciaquarium.com/, then pick "Pricelist", or click link here
http://cciaquarium.com/price.html. The prices you see are wholesale, but don't worry, the retail price is not much higher. Look on list for "One Piece Frame", then your size. You might be 30x12 or 36x12, or 48x12 ?