I don't understand

Hey, I'm 15 and honestly I'm here just to rant, for nearly a year now I have changed so much, I used to go out with my friends have fun and enjoy myself however in the last year I have had more problems with friends etc however everything's fine right now, but now every time I get invited out I just turn it down, I don't want to go out and I don't know why, this has been going on for months and the amount of excuses i have made is insane, I just don't get bored at home ever and I can't have fun and if I want to have fun I have to force myself to pretend, I could honestly get locked in a room for 24 hours and not care on bit. My friends have noticed and keep telling me I need to live and I'll regret it however it makes me more angry and makes me want to stay in more, i want to have the energy to go out but im just never in the mood. I've had problems over a couple of years to do with my dad, he had cancer and now he's an alcoholic and won't listen to anyone, I try and keep that separate from my social life however i think it may have effected me without me even realising, my friends know about my dad but they don't know how serious it is, they all have problems of their own and see therapists etc and I feel like if i see someone about how i feel they will feel like I'm attention seeking or copying them. I don't want to trouble my mum because I feel like she has a lot to deal with anyway seeing as she's divorced and trying to work for us to live. I'm just honestly so down and I just don't know what to do, I don't want to bother anyone with my pathetic feelings and I don't know what to do, sorry to rant I know no one has probably read his but yeah I needed to get it off my chest.