Honestly I think everything you are feeling right now is completely justified and very normal.
I also think that the shock hasn't sunk in yet.
Yes, what you have mentioned and I wont lie to you, it does sometimes become a struggle, and yes your elder two will have to sometimes wait, your attention wont be divided by one new newborn, it will be another two newborns both who has different needs at the same time and unfortunately for them - they will also need to wait and learn to be patient - because you are and will be even more outnumbered and only still have one pair of hands.
So yes, it will be tough, it will be a struggle at times, of course it will. Will it be worth it - for me, yes. But for you, only you can make that decision and only time will tell.
I do really hope that you reach out in real life and speak to someone, tamba do run a twinline and I have used it a few times especially when the twins were very young and it was double the crying/teething.
It wasn't all roses for me, I had very bad PND. I was frogmarched to the GP by my husband and my mum had one baby and my sister took the other so I had a break for a few days. I didn't hear anything but screaming and crying even when it was silent. I felt like I was going insane. I think that's why I am enjoying them so much more now they are a lot older and more independent. I am getting my life back a little now and energy levels.
There are many many many multiple mums who feel exactly the same as you do right now. So don't feel awful about wishing one away - I only wanted one aswell and it took me a long time to admit that - and an even longer time to say it out loud. Especially with everyone elses comments about double trouble, hands full, two for the price of one (ha ha bloody ha) and asking if they are natural, ivf, section, breastfed, and then the other children comments - do you even have a tv? - that's a firm favourite in the fb group im in!
How old are your other DC's?
Please join the multiple mums fbook group im in - please - you will see how we each cope and share our tips and tricks and ideas in having twins - the highs and lows and all in between - knowing and seeing others in your position may really help you wrap your head around it all and start looking forward rather than dreading it.
xx I wish you all the best xx