I've sperm banked, but knowing your vital reproductive system is being chemically eviscerated and physical proof of this is now happening (size and softening of the 'jewels') is depressingly sobering.

"What the HELL am I doing..." has crossed my mind often today. Once I step over the cliff, that's all she wrote.

It's the first time I've really felt unsure about things especially since I'm the last link in my lineage chain.

The stress I'm going through is unbearable to say the least...

I think this is normal when one under takes something of this magnitude. I have doubts on occasion, but I think of what life was like before and they dissolve. I am much happier and healthier than before and it fills me with strength and resolve to continue the road I am on. My Therapist has been instrumental in getting me to this point in my life and helping me over these hurdles. Just stop and think if you could go back to life as before and if you could ever be that way again. 

I can't speak from experience, since I'm still pre-HRT, but I've been thinking about the topic in relation to just our human biology. Our mind may be the wrong gender in whole, but structurally there would be specifically male functions in our brains (mtf) that are primarily concerned with reproduction, and those sections of your brain are probably going haywire. It doesn't know your goal, you, personally, have the goal in mind, and so you need to try and calm down the less rational functions that are panicking on behalf of your birth gender.

Personally, I never wanted children, and yet, the idea of becoming sterile is terrifying. I blame it on these biological male functions, but I know what I want so I can override that panic. I hope you start feeling better soon <3

As long as you have reproductive material stored, you have insurance for having your own biological child one day.  However, it's definitely a challenging thing for a lot of people going through this.  If you're feeling unsure, you can slow your transition down, put it on pause, or maybe see a gender therapist and continue from there.  Weigh your options.  There are so many!  You're not trapped in any decision you make or don't make.

I never had kids, nor can my brain (due to injury) deal with the drama and noise levels that come with babies and young children. so when it came time transition it wasnt even a thought in my mind nor has it cross my mind of my own accord.

I've been given the liniage talk... from a dying grandparent no less. I am the last "male" who can pass off the family name.
Know what I have to say to that?
"Eff that."
The family is messed up. We are all paranoid crazy hermits who can't tell their ass from a hole in the ground. I'm the first to stand up and be myself, instead of hiding in the woods for the sake of everyone else's comfort because "I'm weird." I'll be damned if I go back to that lifestyle, just to waste 20+ years of my life teaching a little one to do the same. I'd rather be out here, in the real world, being proud of who I am, and not giving a crap what shallow people think of me.
I'm me, and I'm proud of it! Which is more than I can say for the rest of my family.

Sounds like a good reason to have a talk to your therapist and discuss how you are feeling.