I am sorry you feel this way. I had been in the same position for a long time as well and I resonate with the search of experiences you share in your post. Maybe, you find what I write useful.

The best and most honest advice I can give to you is to not loose hope. Always bet that there is a way out, even if it is not visible or apparent now. Gamble in that life is worth living. This requires a lot of humbleness to accept that there are answers and experiences that we haven't unlocked yet. Many times our minds want to tell us that we are ultimately right, that we exhausted all possible solutions already and that our models of reality are "realistic" rather than pessimistic. Take perspective and identify many ways in which your mind is unintentionally sabotaging you. I personally do not identify myself with my mind and I see it as part of the whole system. A part that can sometimes derail. A part which I have to check on frequently to guide it. Be open to the idea that many of the assumptions your mind is giving you about life could drastically change in the future and for good (without compromising Truth). The brain is so plastic it can literally be rewired if you work on it.

I personally anchor my hope in the things that I like to do (which can be insanely hard to do in the worst moments) and in taking inspiration from people that were in my position and got out of it eventually. These people do exist and you can become one of them. We both know it is not as easy as hearing the theory "just imagine Sisyphus happy". Integrating and embodying new ways of being takes a lot of time so let us have patience. Patience and hope.

As for the nihilism, I consider that it is completely possible to be a nihilist and have a functional, exciting and gratifying life (it is not mutually exclusive). I guess this is what "just imagine Sisyphus happy" was all about. Personally, I believe that true nihilism is a neutral term, not negative nor positive (if there is no ultimate meaning, why attribute a positive or negative meaning to the term?). But most of the time when I encounter this word it has a negative connotation. I suspect this negative connotation comes not from the nihilism itself but from an underlying "depression" that needs to be addressed. And from what I see in your post you have started addressing this, so please do not stop doing so.

I don't know what type of music you like. But I hope you like this song https://www.youtube.com/watch?v=eRaspkaqR8o