The worried feelings you have are completely normal so please don't feel guilty about them! I think we have all had our 'I wish I didn't have twins moments' or I certainly have but then I see my girls and know that I wouldn't have it any other way.
It IS tough and it's bloody hard work but you're an old hand at this mothering business and I bet you will surprise yourself with your capacity to cope!
I'd suggest you set up as much support in advance of the birth and for the later stages of pregnancy as possible. Don't be afraid to ask for help and enlist the support of friends, family, your older DCs, etc.
Twins are really, really special. You will bond with both your babies. It's just twice as much love which is overwhelming but also wonderful! Honestly, cuddling two babies at once and looking from one to the other is just indescribably lovely. I only have my DDs but if I had a singleton I'd find it odd not to have both my arms full of snuggly babies!
Other lovely things....
Watching newborn twins sleep curled around one another when they see one another as extensions of themselves.
The first moment they notice they have a twin and that this is someone else's arm/leg and not their own. The first time they really look at one another is amazing.
Hearing them giggle and chatter when they go to bed at night.
The way they look for their twin if she is not in the room.
When they learn to say one another's names.
When they dance together.
When they cuddle each other and stroke each other's heads and say 'awwww'
Two little people flinging themselves at you and calling for you.
Feeling like a superwoman as you parade your enormous baby-filled pram around much the admiration of all.
Massive congratulations Heighho and good luck!