Mazal tov on your upcoming wedding!
We appreciate the sensitive nature of this question.
We are sorry to hear that you felt uncomfortable and consequently lacked the kavannah that you had hoped for. Rest assured that this type of lack does not affect the validity of immersion. Even so, you are permitted–if you’d like–to arrange another immersion before your wedding, without a beracha or attendant, making time for any kavannot in the water.
It can take time to adjust to mikveh immersion with an attendant, and for some women it is not comfortable. In the future, you could ask that the attendant stand as far as possible from the water, just close enough to see that your hair is fully submerged. You can also request in advance that she leave you some time alone in the water after immersion. If you prefer it, you can also arrange to have the halachically observant Jewish woman of your choice accompany you.
We hope that these suggestions help. Please don’t hesitate to be in touch going forward to follow up on this or to raise any other concern.