Until i'd read the last bit you put on i was going to say it sounds like you're a bit depressed. Not sure now though as you sounded much happier in that last bit!
If you are depressed though then its probably not the fact you are an only child but that everything in your life seems bad. You could still feel like this even if you did have brothers and sisters. there are deffinately pros and cons to having them or not. i love my brothers very much and feel lucky to have them but i've felt like killing them a few times in my life!! Its very sad that there are people on here that dont speak to their brothers and sisters.
I have also been through very bad times and depression. so i know what its like. It does get better though and you can help yourself, like you said friends and cousins are just as important sometimes easier to talk to. try to sort things with your mum as thats one of the most important relationships in your life. if you cant talk to her then maybe write her a letter, telling her your feelings and that you understand hers. try not to blame her for you being an only child. i hope it all works out for you. youre young so enjoy life, have lots of fun, positive thinking works and remember life is what you make it!