Can anyone suggest some exercises to help increase mental submission? (without feeding sexual fantasies?)
Create a list of non-sexual things that you don't want to do, and then work on mechanisms with yourself or partner to do those things.
ways to (truly) honor my Mistress wishes and preferences without getting distracted by selfish thoughts
Perhaps this is a start....but I think the trick is that it needs to be her list......and I need to truly want to do them without being distracted by her beauty or my needy thoughts and tendencies after prolonged lock-ups. To be specific...I have come to terms with never being allowed intercourse again, but I still struggle whenever I see her with wanting to drop to my knees and taste her. This is not her wish it is mine and entirely selfish. If I am able to become truly mentally submissive I should not want to even think of something unless it is her of her choosing.
There's nothing wrong with wanting to drop to your knees and please her, that's not selfish. It's only a problem and selfish if you constantly pester her about giving her pleasure when she doesn't want it. It's not about the kinds of sexual thoughts you're having but more about the ability to behave yourself when you're desiring her the most and just do as she wishes. Chastity wouldn't work very well without the sexual tension and desire, that's what makes it most fun. If you live and accept the mindset that things will only happen with her approval you don't need to worry about anything else. Just carry on in the ways that make her most happy and you will be rewarded if she truly appreciates your efforts.
I think this is the best advice yet and one my Mistress would approve of as she is only interested in a service submissive while I am still a slave to my perceived "needs". I'm ashamed to be such a typical male when I have such a wise (and beautiful) Mistress to focus on serving.
There are other ways to show intimacy sans penetration. I presume those are still good game and great?
Think of something that your partner has done that has angered, annoyed or upset you, and forgive it. Forgive completely in your head because you are the submissive and They the Dominant
Ask her what she wants - not sexual, just things to make her life easier/better - and then give her them. Good luck
I admit that sometimes it can be hard to focus solely on the needs of your partner, but just keep repeating to yourself how much of a privilege it is to be chosen by your partner to serve her. There are a lot of men she could have chosen, but she chose you. Be grateful for that, and do the best you can to stay focused on her needs.