I tend to think SamanthaD has a bit of a point, although I wouldn't say it like she did.
TheLoneWolf, when I came out to my Mom, the first thing she did was act a bit shocked, but she was supportive. A couple of months later, she became unsupportive, and compared my transition to the "sin" of having a child out of wedlock. Then, she became tolerant of my transition, neither supportive or unsupportive, but making sure I was included in family gatherings. In the past six months, she started using female pronouns to describe me.
These changes in my relationship with my Mom have been developing over five-and-a-half years. I look at it this way: I had a lifetime to get used to the idea that my sex and gender didn't match at birth, and she got only three months notice that I was going to transition. She's now had five-and-a-half years to get used to me being Autumn.
I guess what I'm saying is I'm concerned that you're mentioning you felt "high" when your SO gave a supportive response. My guess is that a day is coming when she asks you "Are you sure you want to do this?", or she refers to you with male pronouns a few times when you are presenting as female -- or some other such development. Will that be a reason for as much as an emotional low as her support was a reason for your emotional high?
I guess my comment is that I'd recommend a longer view perspective than the current moment to judge how your SO's and your relationship is going. Expect rough patches in your relationship with her (as well as with friends and family) will occur due to your transition. Whatever good or bad days you may have ahead, none of the days are going to be the "end of the world." Enjoy the highs, but expect some lows. Your transition isn't yours alone: Everyone who maintains a relationship with you -- SO, family member, or friend -- is in a sense transitioning with you.
Even with those who accept and embrace your transition may take awhile to fully absorb your transition as a permanent thing. And, they may go through some of their own transitioning process -- much as my Mom did -- with their feelings about your transition changing from day to day, or week to week, etc., for quite awhile.
So I guess my basic comment is this: If your SO's feelings seem to change in a few days -- and then change again in a few days again, and then again, and then again... -- well, don't sweat it too much. Even though she may end up being as supportive as she was in your conversation broaching the subject of transition, she may waiver a bit in her support before sticking to that full support. But even if she were to become fully unsupportive -- don't get too low...honest to goodness, the world won't end.
And, an added comment is that if you have a therapist involved in your transition, you might want to bring your SO in for at least one of your therapy sessions. Getting an outside look at your relationsip to possibly help your relationship from degrading wouldn't be a bad thing either. Heck, even getting a mutual friend you both respect involved in discussing relationsip issues in the same room as you both might not be a bad idea either.
_________________________
~~Autumn~~
As if there were safety in stupidity alone.
--Henry David Thoreau