Do women wear a tallit while praying? I do, as well as sisith - even to work. I find myself holding them in my hands when I feel unsure or insecure. I helps me remember that HaShem is in control. Is it wrong for me to do so?
Torah observant women do not usually wear a talith during prayer, or ever for that matter. Many Reform women do; but they do so more as an act of feminism rather than an act of devotion. Consequently, even if you wear a talith as an act of devotion, many Jews are likely to interpret it as an act of defiance against what liberals view as sexist primitive religious norms.
Despite this, it is explicitly stated in the Mishneh Torah (Hilkhoth Sisith 3,10) that there is no prohibition against a woman wearing a talith or doing any other Torah practice that only men are obligated in - unless it is clear that only the explicitly qualified are permitted to do the particular act. Concerning the wearing of a talith, however, the halakha states unequivocally that a woman should not be prevented from doing so. This is especially true if she is wears sisith as an act of devotion. So long as she does not say the blessing "...Who commanded us..." before doing the act that she is not commanded to do, there is no error in her observance. It must be noted, however, that a woman wishing to wear a talith must be careful not to wear a style of talith that is associated with a man's style of the garment (ie: the typical white talith with black or blue stripes). Although she is permitted to wear a talith, she is not permitted to violate the Torah prohibition against wearing the garment of the opposite sex. So, if you can find or make a feminine looking talith, that would be acceptable. Only the garment's overall appearance need be different in style from that of men's, but not the manner in which the sisith are tied.
The reason Orthodox rabbis discourage women from wearing a talith even though it is technically permissible is because women are not obligated to put sisith on four cornered garments as men are -- even though they are allowed to. The concern is that, as we see with the Reform movement, many women will begin passionately keeping certain commands they are not even obligated in, simply to assert their equality with men, while not giving priority to those commands that they are actually obligated in.
On another note, wearing a talith will enable a woman to cover her head while praying, as is mandated while praying the Amida. Additionally, for those who oppose the wearing of a radidh due to its similarity to Muslim dress, placement of sisith on the four corners of the woman's head-covering would give a unique indication to the woman's Jewishness, thus resolving this particular objection to a greater level of modest clothing.
No one should be prevented from doing permitted acts that strengthen their sense of connection to G-d and walking in His ways.
R' Yosef Eliyah
(Laws of Fringes) Hilkhoth Sisith 3:10 
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