Socialising on the forums helps me to get more used to socialising, because I've lived quite an isolating lifestyle all my life. Socialising has always been a huge fear of mine. I'm already making large improvements. I don't know who I am anymore. I used to be so withdrawn even online and now I'm taking up tons of the Osu! forums and potentially making a friend. I have help with socialising in the real world as well and going to places I would usually find a complete nightmare to even think about. The forums do help a lot with this, too. I find myself overthinking what to say less and I realise that people aren't usually as horrible as sources such as the news try to make out.
As for the game, feeling that I've done well makes me feel good about myself. I have very low self confidence and always felt that I don't deserve good in life. I'd rather do poor in my GCSEs and know that it made someone else do good because it's a competition sort of thing, than to take the good scores I feel I don't deserve due to my poor mental health, and this inner conflict drives me crazy. This sort of thinking is pretty depressing. Spending ages trying to complete a map and finally passing or FCing it gives me a huge boost! It's possible to be happy about progress while not thinking of it as a competition due to the single player mode. I'm hoping to maybe have more confidence with competitiveness in the future.
TL;DR: Osu! is pretty therapeutic.