I'm an only child too and yes, it screws you up for life.
Even though my parents decided early NOT to spoil me... (Never had a new bicycle, had to do yard work with Dad to "earn" the money to buy a second hand one)...emotionally, I didn't have to share their affection with brothers or sisters. I didn't have other kids to play with and my mindset is rather different. I have noticed I do the following:
*I'm not afraid to be alone. I was raised alone, I know how to amuse my self and if you become a problem, I'll dump your ass and never look back. I'm not afraid to be alone, but I'm terrified of getting hurt.
*I am independent. There was no sister to blame or brother to ask for help. I did it on my own. Did it before you came into my life, will do it after you're gone.
*If you DON'T show me attention immediately, I'll figure you aren't interested and move on. The 3 day rule, not answering my calls/texts, all seems very childish to me. I want to date a grown man, not a boy.
*I am amazing at manipulation. I was raised, not as a child, but a short adult. I learned absurdly early to watch adults, their behavior, hear the inflection in their voices and figured out how to to act with them to be accepted in the group. (Yes, I was that precocious 8 year old at the party with the martini glass full of soda pop discussing the future implications of Watergate.)
*Growing up, I was great with adults...BAD at other kids! HORRIBLE with other kids! They were too childish for my maturity level. I didn't have close friends till about age 11 or 12. Even now I have friends in their 50's & 60's and in their 20's, very few my own age.
*I make a few very close friends that become my pseudo sisters & brothers. Family is sacred to me, and I will walk away from dinner with the Pope if my "Family" needs me.
*I get jealous when my boyfriend shows attention to other women (By "jealous" I mean icy glares, vicious condemnation and hours of pouting) If I think you're doing it on purpose, I'll dump you. If I think you're a player who CAN'T control your flirting, I'll dump you. I WILL protect my heart at any cost. Remember, I'm not afraid to be alone, only of getting hurt!
*I'm drawn to men with large families. If he's got 3 sisters and 4 brothers, 10 aunts, uncles and both sets of grandparents, I'm in heaven. This replaces the large family I craved & didn't have as a child.
*I get along well with first borns & other only borns. Be strong, be decisive, be a leader, or I WILL take the reins from you and make you my b!tch. (Remember, I've been around grown-ups all my life. Weakness to me means "child to be taken care of")
* If you do manage to get through all the land mines I've warned you about, you're in luck. Because I didn't have to share my affection with brothers or sisters, I focus all of it on you. You will be the most spoiled man on the planet. I will notice every glance you make, read every facial expression, sense every mood you're in. I'm SCARY intense with my half of the relationship.
Stare too long at that cool-beyond-words leather jacket at the store? You'll find it hanging in your closet one day. Bummed out because they sold out of tickets to a concert you wanted to go to? Don't worry. I have a battery of friends trying to win them off the radio and several more "people" talking to "other people" about getting tickets on the sly.
The down side is because I'm SO invested, any rebuff from you is devastating to me. Hear that? ANY REBUFF FROM YOU IS DEVISTATING! I am accustomed to total love and acceptance from my parents. I expect it from you as well. (Not logical, but who said love is logical?)
Hope this helps you.