RAW implies that you can put an end to the concentration, but a fragile argument can be presented against
According to PHB, it is very easy to end the concentration.
You can end the concentration at any time (no action required).
In addition, the Polymorph spell indicates:
[The target] keep its alignment and personality. […] The creature is limited in its actions by the nature of its new origin and can not speak, cast spells, or take any action that requires hands or speech.
Here, nothing says that you can not stop the concentration, which seems to be a simple mental activity.
the a The argument I can see is that the beast could not have started to focus (part of the spell casting), but that would imply that the beast can not maintain its focus either, which would never allow you to to succeed in polymorphising you. If the DM forbids to end the concentration, he / she should forbid to polymorphize.