Hi OP, I've been where you are. The thing that helped me the most to come to terms with my grief over not having the family I'd imagined and having so many failed IVFs was a book by Tracey Cleantis called the Next Happy. I'd really recommend it. It really helped me to move on with my life.
As it turned out, after reading this book, grieving and starting to move on I fell pregnant naturally (after about 15 years of failed IVFs). However, it wasn't some magical fix. My children are 7 years apart. They can't play together much, their ages are too different and their interests too. It goes through this in the book, but dreams are usually quite specific and sometimes when you get what you think you want it really isn't the same as you imagined.
I love them both, I'm happy I had them both, but what I've ended up with really isn't my 'dream', what I wanted or what I imagined (didn't imagine being a knackered mum in my 40s for a start). Like most things in life there is good and bad. Some days I feel bad because we could have afforded private school for DD1 if we'd stuck at one, and in terms of education and opportunity that would have been better for her. Sometimes I feel guilty because I'm an old knackered mum to DD2. But they're both loved, and I can't change what's happened- all I can do is my best going forward and love them a lot, which I do.
DD1 used to cry about not having a sibling. In reality, she hated DD2 that first year. After having all our attention for so long it was hard. Children when they imagine a sibling imagine a child that will do whatever they want and not take any parental attention away from them. They don't understand what they're asking for or crying for, not really. Now DD2 says she wants a baby sister like her friend. I suspect if we did have another baby then she'd go through the same difficult time as DD1 did.
It's entirely legitimate and fine to grieve the family you wanted and haven't been able to have, but it's not going to harm your child being an only child. They are clearly loved. That's the most important thing.
I totally understand the jealousy when seeing other families who seem to have what you want... I used to feel that way myself, but I've learned that often their lives are difficult in other ways, and they may well themselves have unfulfilled dreams too (not all of them will, but some of them will).