She has told you it is not boring and flat to her. She has told you she is in it for the long haul. By your words, she has done everything she can (almost) to help you heal.
You think it is boring and flat, that doesn't mean she does (see above).
Couple of thoughts: comparing yourself to the AP or your marriage to the affair is normal. Totally, completely SUCKS but we all do it. But again, those are your thoughts and feelings, not necessarily hers and they are driven by your trauma and resulting nose dive of self esteem. Focus on you. How can you build up your self esteem so that YOU believe you are enough and worth it? Because trust me, once you start to rebuild your love for yourself, the power of that will make these thoughts subside. Bonus: your fear will start to subside also.
Second thought: There is no comparing an affair to a long term marriage. One is more exciting, for sure. The other, however, is deeper and more meaningful. Wouldn't you agree? What would you value more, the thrill of a first kiss or your family? We all like the thrill. It comes down to what we value more.
Serial cake eaters seem to value the thrill above all else.
Most folks, cheaters or not, value the deeper relationship more. Cheaters "forget" this, think they won't get caught, get caught up in the thrill / fantasy, or whatever but some do, when faced with consequences / reality, choose the thing they know they value more i.e. their marriage.
She seems to have made that very choice. Perhaps it was hard for her at first, detoxing off the thrill. But at this point, I'd be truly surprise if she was still pining for AP. She may be realizing just how deep this cut went, what she lost and can't regain (your old marriage, your blind trust) and that might be getting her down.
Talk about that, the two of you. What was lost, for both of you. Then talk about what you would both like the future marriage to look like. Start dreaming, planning and then doing. Neither of you want to remain in the broken marriage. Build a new one.