A lot of recommendations have been posted in other threads. I suggest searching and reading those threads. What you experience is very common, and there is no simple easy fix. All players have these issues, manifested in different ways.
I have previously recommended "Golf beneath the surface" as a good resource. You need to invest the time and effort to train your mind, just as you invest time and effort into your physical skills.
To answer point 3, it is probably because of the pressure you put on yourself to perform like you did previously, and you are also possibly attaching your performance to your self-worth. When combined with an absolute mindset ("I should be able to do this shot"), all hell then breaks loose when you inevitably make mistakes - your performance challenges your thoughts about what you should be able to do, so are essentially attacking yourself with doubt and negative feelings of the self. You are flooding your mind with these, which means you are now essentially multi-tasking between those thoughts and table tennis. Your focus is now split. It's not going to go well and will just spiral out of control.
These thoughts need to be replaced with something more flexible, such as "I would like to do this shot", as this doesn't pile pressure on yourself if you don't. Your performance needs to be disconnected from your self-worth. You need to recognise those thoughts as they are popping up, so you can change them. The above mentioned book has exercises that help with all these things. Once you do the written exercises, it's much easier to be self-aware and make a change. But you can't just read a forum comment like this and expect changes, you really do need to do the work.
I have previously recommended "Golf beneath the surface" as a good resource. You need to invest the time and effort to train your mind, just as you invest time and effort into your physical skills.
To answer point 3, it is probably because of the pressure you put on yourself to perform like you did previously, and you are also possibly attaching your performance to your self-worth. When combined with an absolute mindset ("I should be able to do this shot"), all hell then breaks loose when you inevitably make mistakes - your performance challenges your thoughts about what you should be able to do, so are essentially attacking yourself with doubt and negative feelings of the self. You are flooding your mind with these, which means you are now essentially multi-tasking between those thoughts and table tennis. Your focus is now split. It's not going to go well and will just spiral out of control.
These thoughts need to be replaced with something more flexible, such as "I would like to do this shot", as this doesn't pile pressure on yourself if you don't. Your performance needs to be disconnected from your self-worth. You need to recognise those thoughts as they are popping up, so you can change them. The above mentioned book has exercises that help with all these things. Once you do the written exercises, it's much easier to be self-aware and make a change. But you can't just read a forum comment like this and expect changes, you really do need to do the work.