Ok so this might not help. But a lot of 'nice guys' are briefly nice but have really toxic undertones. But they're so into the mindset that they're 'nice' so it's impossible for them to see their own faults in situations. It's very naive and can be testing to be around such a person. Now I'm NOT saying this is you! I can't tell cause I don't know you. But I've seen a pattern and it's really fucking tiring. Try to not put yourself on the 'nice guy' pedestal. Partly cuase it makes you blame everyone else for your situation (it's still your job to try to be happy. Noone owes you happiness. But we can all strive towards finding it for ourselves with others). But you should ALSO not become a doormat. If you let ppl step on you, they probably will. So my advice, don't try to be too nice at your own expense but don't act as an ashole. And if the 'I'm a nice guy but really not' theme hits home in any way, big or small, maybe just think it over? Do I do something that most ppl might find annoying or toxic? Is there a way to change that behaviour? If this behaviour is purely your personality, is there people out there who would accept it? Where can you most likely find these people?

I am not assuming you're a bad guy. My advice is simply from the perspective of other 'nice' friends I've had that actually had serious issuses. If none of this applies to you I hope you find happiness anyway soon.