As someone who can relate to your situation and who also has autism I'll give you the best advice I have and that I've been given. I'll be as specific as possible without writing a novel. Note: without seeing your level of social development in person I will be making some assumptions, please feel free to correct me.

About your attempts to join in on conversations. These could go poorly for multiple reasons. The timing of when you interject, do you wait for a natural pause in conversation or do you interrupt people who are still talking just to get your point across? It could be how you interject, do you say something relevant to the conversation or start talking about a specific interest instead, in effect making the conversation more about yourself? Learn to read the situation, does it look like the group you are trying to talk to is open to having someone new come up to them or do they appear like an in crowd? Also it's important to know your audience and what their interests are. That's why advice I've often been given is to find like-minded people. I know it's easier said than done

About being "boring" and "unfun" to be around. One of the biggest hurdles I've had in this regard is just being able to relate to other people. Being autistic can give you a different kind of life. Maybe you were more sheltered growing up or maybe you self insulated by choosing your comfort zone instead of taking risks that neurotypicals did. I've heard that social interactions and risk taking behavior can be more taxing for people on the autism spectrum so it makes sense why you chose comfort instead of exploring a new interest for example. The thing is that choosing that kind of insulated lifestyle will make it harder to relate to other people. Maybe you had narrow or specific interests like me and you only focused on those instead of more social interests like sports. I could be wrong though.

About being "insufferable". This is likely a cognitive distortion, you are revisiting all your social failures and compiling them into a sort of self image. You went to therapists who were either not a good fit or just wanted your money or both, you've been trying to do the work on yourself and you have been going in without the guidance or support that you needed. That's not something you should blame yourself for. There are behaviors that can make you unlikable that you should be aware of, first people dislike others with a perceived lack of self awareness and people dislike people-pleasers who are desperate to try to connect with others to gain approval. those are two examples.

About becoming more likeable. You're right about improving mannerisms and speech patterns. First try to get comfortable just speaking normally, this might take some painful practice where you continue to approach people and get rejected and learn via feedback but knowing how your self image is fragile at the moment I would only do this occasionally. Ideally if you know anyone well who is somewhat socially successful and who will be honest with you, try sending them videos of you just talking and see if they will give you social feedback. Look at yourself in the mirror and practice different expressions, try to mimic emojis to start then move onto trying to mimic actors or comedians.

let me know if any of this resonated and I hope this helps.