I'm so sad, and so sorry you are going through this.
In asnwer to your last question, you will not be left on your own. At 21 weeks, i'm sure they have discussed this with you, but you will need to go through an induced labour.
As for if you make the decision to terminate, or not. You need to ensure you are well informed of exactly what the malformation is. How is it compatible with life. If you decide to go to term to have her, will she be in pain, or suffer, at birth. they need to tell you more on things like, is there any chance at all she can live, or survive with the malformations.
I feel that until you have as much information as possible, and enough knowledge on the problems you baby has, you cannot make such a huge decision.
I feel so deeply for you, and cannot begin to imagine what you must be feeling now, or going through.
Should you decide that it would be the best, kindest, most caring thing you can do for your baby girl, to end you pregnancy, then the hospital will be there for you. you will not be left alone to have her. There is a lot of support there for you afterwards, and also many fantastic organisations also, who are mainly run by people who have been in your situation and can access and guide you to the right people and right places for help and support to come to terms with what you are going through.
I hope i've not been to insensitive, I just felt that you needed a little more information.
you are the only people who can make such an agonizing decision, but I truly feel you need more knowledge of what you are facing, either way, before you can make such a choice.
You will be in my thoughts and prayers so much. I just feel so terribly sad for you.