Go sky diving. Changed my life forever.
The problem here is that you are afraid, which is normal because we are all afraid at some time or another. We have many reasons to be. But what is important is that you face that fear and realize that it is simply fear that is causing these feelings in you. if you can take a step back and look at your thought process, and then tell yourself "its just fear." you will see how your world changes.
Its not an easy journey, and like any other life it will have its twists and turns, but you have to stop putting your life on these pedestals based off of you are "expected". First, you need to realize that what you are "doing to your girlfriend", isnt wrong... you aren't doing anything other than being the person you were meant to be, and if she wants to stick around for that then you need to see it for what it is and realize that not many significant others stay. I obviously don't know about your relationships situation, but if your gf is struggling, id bet that its because shes struggling with her own feelings of loving you and not having an easy time dealing with it all... and you have to realize that this is better than what a lot of people get... she is having an internal fight instead of just taking the easy route and walking away.
Im not exactly sure what you mean about having the character to survive transition... I think you might be over-generalizing what it means to be a transgender woman 0_o I mean, just as there are all types of cis-people, there are all types of trans people, some are emotionally strong or physically strong or sometimes both, some are extroverted and others are introverted, some want to make an impact, others just want to blend in. There is no specific "trans character"... Transition isn't easy for anyone, its just that everyone has their own way of dealing with the adversity that they run into.
Its also worth remembering that your mind is your worst critic/judge/bully/etc. the only way to ignore it is to change your mindset.
I use to hate myself for being trans... I felt like i was burdening my wife, family, brothers, friends... I wanted to kill myself because I felt like the thing that made me normal (being male) was the only reason everyone around me felt like they could continue being part of my life. But I was so wrong... I look back now and I feel like having begun transition and embracing my true identity was the best thing that ever happened to me. Sure, im crying all the time, I feel a lot of pain sometimes and sometimes I fear that my future will be bleak... but to go back would be like jumping back into the 9th layer of hell... There is absolutely no feeling like waking up and actually feeling like your life is worth living. In 24 years I've accomplished a few things, but never felt any pride, only shame in myself... but THIS... even if 99% of the world disagrees with this, I will forever be endlessly proud of myself for it, because its not every day that people decide that they are going to chase their happiness at the cost of everything around them.
You need to start living for you and stop living for others, because the moment you start doing that you will see that others will follow. Energy is a beautiful thing, and I thought I had it all before transition; I took the leap and chose to take the path that could have resulted in complete shunning.... but because my depression is gone, my anger is gone... I am living MY life TODAY for ME and no one else... People are drawn to me now... even the ones who can't bring themselves to understand. My wife and I had HUGE arguments about my transition... but 3 months later, she sees and FEELS my happiness... and it has made our relationship better than it has ever been in our first 5 years where I was male... Don't give up and keep trying to see the silver lining, because energy works both ways... if you allow yourself to be encompassed by negative energy, you will be surrounded by negative energy and will attract unwanted amounts of it... but i can guarantee you that the moment you start living for you and stop feeling guilt over your choice to be happy, you will see that others will feel happy with you and for you; you will see that you had no reason to feel guilty.
Don't feel guilty for what your GF is going through, because if she is sticking around and having a hard time with it, its because she feels guilty about leaving someone she loves over aesthetics. Give it time and allow yourself room to breathe, you'll be alright.
Sorry for the long response, i have a tendency of doing that =/ but Even if none of what i said makes any sense in relation to your situation, i hope it helps in some way <3