For what it's worth...
When I started dating my now-husband, he was an active Witness. He stopped going to meetings soon after we got together, not because of me, but because he finally felt like he didn't HAVE to, like there was someone to support him outside of the congregation. His last girlfriend was a Witness, but they were a match made in Hades and it was disastrous. So, he wasn't strong in the "truth" to start with, and our relationship was very, very normal. (Planning the wedding - now, that was another story.)
So, I guess I'm saying, I don't know what it would have been like if had been strong in the faith. But when we got together, he looked (to others) like he WAS. I, of course, was blamed for his fading, but we both know (and I know from his writings before our relationship) that he was mentally and emotionally out of the religion long before we fell in love.
If the person you are dating IS strongly in the faith, then you will have a harder time. But if there are doubts or if the person wants to live outside of the religion, then I think there is a greater chance for success.
Best of luck to you .