Edward's childhood was not a happy one, not in the least. There were bits of happiness here and there, but for the most part, it was one that was spent achingly alone and ignored. He had a father that saw him only as a means to an end. He wasn't particularly wanted by his father, the only reason he was even here was because his mother had wanted a baby.
Liam Connoly was a charismatic man and as soon as Maggie laid her eyes on him, she was completely under his spell. He became the center of her universe, as exemplified by the fact that she turned her back on her family and everything she knew, to go with him back to Ireland when she was just seventeen-years-old. To say she suffered a culture shock when she arrived in Ireland would be an understatement, but she loved Liam too much to complain. She thought having a baby would make things better for her, give her something to do during the day when Liam was at work, so after they had gotten married, she got pregnant.
Things were better for Maggie for a time, and Liam even seemed excited by the prospect of having a child, once he found out the baby was going to be a boy that is. By the time Edward reached age five, Liam had grown weary of Maggie's attention to the boy and began demanding that she pay closer attention to his needs instead. Faced with the thought of losing the love of her life, Maggie did as he asked.
From that time on, Edward was little more than an afterthought to each of his parents. They were so consumed with each other that he was often left to fend for himself for most everything. Getting himself to and from school, making his own lunch, fixing his own dinner...all of his basic needs, he had to tend to himself.
Edward had learned early on that no matter what he did or how well he did in school or how hard he tried, he didn't matter enough to his parents. They barely acknowledged his existence. Sure there were times his mother went out of her way to do something nice for him or spend time with him, but those times were very few and far between. She was just too focused on making his father happy to be much of a mother to him. By the time he was adopted by Carlisle and Esme, Edward had vowed that he would never feel unworthy again. In order to do so, he figured it was best to keep to himself and not get too close to anyone. In his mind, if he kept his distance and didn't get too close to anyone, they couldn't hurt him the way his parents had.