for you, I am with HotI - give yourself some time and see how you feel.
as an only child, I didnt have any resentment/issues/problems. i was surrounded by friends and family who i was very close to and mum always took friends on holidays, let me have friends stay etc. i never knew any different and i didnt and still dont think i missed out. its a different experience to having siblings of course, but every family is different.