First of all sorry if my English is sometimes bad. Im not a native.

I’m thinking since a few weeks that I have depression. I always knew that I was in some way depressive but I thought it wasn’t real depression. Just a little Version of it. Most of the days I’m fighting this little voice in me That says that I’m worthless or nobody likes me or I’m can’t do anything etc. I know that these thoughts are not true. I know that my family and friends love me und would be there for me when I need them. But the thing is that sometimes it’s very hard to fight these thoughts. It’s tiring and a few times it’s tempting to just let them in and believe them.  
Often I think I should talk to Someone about this, because I have this thoughts, feelings since my teenage years in me. Then I had not many friends and I was really self conscious about everything. Nowadays I know that I’m not the only one who feels like that and that it’s pretty common, but I’m 23 years old now and I never talked to someone about that. I don’t talk with my family about this because I know they won’t understand mental health issues. The first question would be, why I am feeling sad or depressed. I don’t know the answer to that. My life’s good. I don’t have big or existential problems. They always minimize my feelings and in my mind it’s my first response too. I always make my feelings and thoughts seem less important. I could talk my friends about it, they would wanna help me or be there for me. I don’t know why but always when I try I have a feeling I shouldn’t. They have their own problems, maybe they wouldn’t understand me, they don’t have time for that (I know that because my friends are the most times busy), and worst case scenario: they just feel pity or they insult me. I know they wouldn’t because they are great people but my mind just always convinces me to let them be. It’s just very difficult for me to talk openly about my feelings and I don’t have enough trust in anyone to be completely there for me. I know I shouldn’t expect from everyone to fully be there for me because they have their own life and struggles and things to do.  
Last week I had a pretty bad day. It was one of these days where this little voice inside my head had won and I was very low or really not the best version of myself. On this day I met my best friend to go home together (it’s a two hours trainride and because of constructions it took so much longer). I really tried to be nice but I think I was really insulting on that day because I was feeling depressed. You must know that how much worse my mood gets my humour gets also very dark. And what from my perspective were sarcastic comments for my friend was hurtful. That explains why she didn’t talk to me the last few days. I just thought she was simply busy because she has in a few weeks exams and she is learning. Today she wrote me that she was very hurt and she likes to talk about last week.  
Now I don’t really know what to do. Of course I want to apologize because even if it wasn’t my intention I hurt her. And I will promise her not to let my bad mood on her out because I have no right to hurt other people because i am hurting. But I am scared of the conversation she wants to have with me about it. When she asks why I was behaving like that I don’t know what to answer. I think I am not ready to talk about my depression to people I know. But she deserves the truth. But also I don’t want to bother her. I know we should talk about what happened last week especially because I hurt her feelings but I’m afraid because I’m not good with expressing emotions.  
And I have a feeling I’m overthinking too, because it’s my best friend and there is a great possibility she could understand me.