My pelvic floor is really strong and always has been. I had a small graze with my first birth but I would put that down to the dodgey laying down position I was asked to get in and the coached pushing rather than my body. It's really important during pregnancy to have a healthy pelvic floor and I have never heard of one being too strong for labour. If I were you I'd firstly just practice all the usual relaxation techniques during birth - visualisation, water, hypnotherapy, birthing in an environment where I feel safe and secure etc. but I'd also ask your doctor to give you further information about having a "too tight pelvic floor" and ask him to show you some evidence of this.

Also, what on earth is he giving you internals for? What benefit or feedback could that give you that you couldn't find out without them? Maybe you just weren't very relaxed when you were having an internal but you will be relaxed during labour. BTW, I think it's been established now through studies that perenial massage doesn't reduce the incidence of tearing but it can be helpful for women who aren't that comfortable with their bodies as a preparation for labour.