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.
First of all, we advise you to avoid whispers as there is no other way to get rid of them. Intention is an easy matter because the meaning of intention is to intend [to do something], and, no doubt, that you perform Ghusl with a given purpose, so this purpose is your intention.
However, the required intention for Ghusl due to menses or due to sexual defilement is achieved if a woman who performs Ghusl intends to purify herself from sexual defilement if she was in a state of sexual defilement, or to remove the state of being impure from menses if she was in menses [and her menses stopped], or to perform an act that needs purity like the prayer, or that she intends performing the obligation of Ghusl or the obligatory Ghusl, or doing Ghusl.
In fact, achieving this intention does not constitute a hardship because the intention as we mentioned means intending to do an act. Since you believe that Ghusl after sexual defilement or after the end of menses is an obligation on you, and you performed Ghusl in order to achieve this obligation, or that you performed Ghusl in order to perform the prayer or other acts that need purity, then the intention is achieved as we mentioned and your Ghusl is valid. The place of intention is the heart and there is no need to mention it by tongue.
On the other hand, if you intended just to clean yourself and did not intend the obligatory Ghusl, and did not intend anything of what we mentioned above, then your Ghusl is not valid and in this case you have to make up all the prayers that you had performed with that Ghusl.
For more benefit, please refer to Fataawa 88124 and 83401.
Allaah Knows best.