The greatest gift you can give your child is a brother or sister I'm sorry, but this is bollocks and I think you need to get rid of this sort of idea, through counseling if necessary. Surely the greatest gift you give your child is a) their life and b) your love and attention.
I'm an only child, and tbh, I have never wanted siblings. Now I am older, I also appreciate all the advantages I've had due to being an only child- including the extra financial help my parents have been able to give me.
Occasionally, I have wondered what it would be like to have a sibling, but among my friends, as many have bad sibling relationships as they have good ones. I know one set of adult siblings who genuinely hate each other, and the brother has admitted to me he sometimes wishes his younger sister was never born. Equally, I know of another family where the older brother resents his younger sister, who he feels was favored over him. I also know still more adult siblings who just don't have a close relationship and never had done. Actually, in general, it's the older brothers I know who are least close to their younger siblings.
Often people talk about the burden of older parents on only children, but equally if there are lots of children with different views on how best to care for parents, or one sibling who does more caring than the others, it can cause a lot of additional stress.
I do think you should seek counseling or some kind of support to deal with all these feelings. I actually think this is especially important if you are continuing to try and have another child, as, at this stage, I think you could put a lot of pressure on the sibling relationship, especially if they end up being not as close as you would like.