My experiences are as follows:
I am female, I can only insert roughly 5 to 6 inches of any dildo, before it hits my cervix. It does not matter how aroused I am, or how much experience I have had (I have been having sex for 15 years and playing with a variety of toys for ten) It seems that my vagina "tops out" at 6 inches. Anything longer will hit my cervix and hurt. Now, penises I can take a little more. Not much, but a little and I think that is because they are generally softer and springier and so they don't poke or jab the cervix as much as toys do.
My partner is male, he has been putting things up his bum for about 2 to 3 years now. He cannot insert more than 4 to 5 inches without it feeling uncomfortable. So, a little science: The sigmoid colon has a kink in it...a sort of 90 degree angle and this section of bowel lives about 4 to 6 inches up your bum (I don't know exactly...I am not a doctor lol) When something inserted, reaches this kink in the bowel, it can, for most people and especially at first, feel uncomfortable, crampy, bruised, achy or whatever other sensation, as the toy tries to meander around this bend, but will put some pressure into the wall of the colon as it does (That is what feels uncomfortable) This is a phenomenon that many men (and women) experience and some men never get past that. They just do not like deep toys, preferring to play below that kink in the colon (Which luckily, is where the prostate is anyway. This is just a few inches inside the bum) Some guys do get past that and become fans of length and inserting longggg toys for some deep deep penetration.
So, the point is, we are all different. The vagina has a roof and unfortunately we just cannot insert 18 inches of sex toy. Most women, from what I can tell from amateur porn and reviews and such, tend to be roughly between 4 and 6 inches deep, before the toy hits the cervix. Some can bang themselves with 7 inches and not touch the cervix, but this is rarer. The dildo on your strap-on is 7.5 inches long and I would say this: If she is struggling to take that fully now (as most women would be) then that won't change. Some days she might be able to take a CM or two more than the day before, but she wont ever go from feeling discomfort at half way inserted, to fully taking it (Assuming it is depth that is the problem here, which I am almost certain it is)
As for you...well it is possible that one day you may begin to enjoy pushing past that point internally and you may begin to enjoy deep insertions, but not all men do and there are lots who never do. I mean....I have been pegging my partner for two years now and he has always, always had to stop me if I go to deep. It starts to cause him cramps and the session is over with.
I suggest getting yourselfs a 6 inch dildo. Maybe even a 5 inch to 5.5 inch long if you want to pound each other in comfort. Bare in mind, this may still be a whisker too long for you. It is all about experimenting really.
Your prostate is merely 2 to 3 inches inside your bum....her g-spot similar inside her vagina...reduce the length of that dildo and I have a feeling you guys will be having a lot more fun with your strap-on. You are correct, girth can be worked on, but length definitely not for vaginal and in a lot of cases not for anal either. You may be one of those men who simply hates that sensation of depth and just need a nice, short toy to play with. Nothing wrong with that.