it would be very difficult destroying something youve built. Many happy times, wonderful memories.
Yes, it would be very difficult. But it will be even more difficult if you make the decision to get baptized FOR HER and then some years down the road realize it was a big mistake. By then things will be built even higher, and will come crashing down even harder.
I don't know anything about your situation, but what's been built can't be that strong if she will ultimately give you an ultimatum to be baptized or that's it. I've seen from experience that if relationships are built with the idea in mind that someone will do something eventually, then it is not a good relationship. Because either the one person will eventually do what the other has always wanted and not be happy about it, or the one doing the pushing will eventually say that's it, you didn't do what i wanted, it's over.
Trust me, getting involved with the Jehovah's Witness religion is a huge step with life altering consequences. You have to be 100% sure it's what you want to do if you decide to do. You can't do it for her, because eventually you will get to a point where either she realizes you didn't want to do it, or you will resent her for making you do it.
Relationships should not be built on any sort of conditions. If they are built on an unsteady foundation, they will eventually crumble. If both people do not have the same things in mind, or the same goals, it will not work.
Think long and hard about this decision, and do not be forced to make one. If she really loves you, she will understand that you have to be ready to make this decision yourself.