You can always find someone who seemingly has more than you/a happeier life/a better life. By the same token you will always be able to find someone who does have what you have.
What helps me when I'm in the throes of the 'poor me's is a gratitude list.
I start:
I am grateful for...
and list everything that I am gratful to have in my life. It helps me shift my perspective from what I don't have to what I do and brings a measure of contentment when I focus on that.
ps. Don't fret over being an only child. Just means your parents realised that it wouldn't be fair to have another child after theperfection that is their first!!