Going way out on a limb here, I'm wondering if it is possible that she is having orgasms, but they are not the explosive, screaming and hollering kind. Sometimes when a woman "becomes ticklish and can't tolerate further stimulation" it is a sign that she has had an orgasm. I realize that she would normally know if she is having orgasms. Daily sex in this situation might be pushing it. At least, I hope you both can enjoy the closeness and the feeling along the way, until this issue is resolved to your mutual satisfaction.
To answer your questions:
1) The problem occurs in every conceivable situation. She has never really experienced orgasms during intercourse. But she did have them during oral sex, from manual stimulation and with vibrators. Now none of these approaches works at all. They all build toward orgasms as before, but somehow the moment of climax is just gone for her; leaving her either extremely frustrated or in tears. She tries to orgasm most times that we make love and it is really hard for me to watch her suffer. We have tried everything from hours long sessions where I stress relaxation, and allow her to be completely inactive, to having her take total charge and completely go wild with stimulating herself. All to no avail. We have experimented with anal stimulation as well. This contributes to the arousal, and she enjoys it, but it doesn't help to push her over the top.
2) Medications - She does take an anti-depressant (Welbutrin), but she began using it several years before she lost her orgasm. In fact, this drug increased her libido and if anything generated more orgasms.
Is she taking any medications.
Is this a problem with both oral and intercourse? May I speak frankly? I would try a relaxed oral session with you being the active one, and her on her back with a lubed finger gently going into her anus a short distance, slowly in and almost out, along with both hands massaging her butt cheeks and thighs. Don't be in a hurry. Otherwise, also, try regular intercourse with her on top, if you haven't already, as you massage her butt cheeks and touch her anus.