it's been said, but i'll say it again, magic is energy found in nature, it cannot go against nature. humans cannot shapeshift or change into ANY other creature. the spells that claim this to be true are put on by people who do not understand how magic works, saw some movie in which people could, and belief it to be true.
what you could try is astral projection. your astral self [think of it as your spirit] leaves your body and goes to the astral plain where you can change into other forms because it's not physical. i'm not sure how this is done since i've never tried, but you can do it. you could also try incorporating tigers into your study, pray to deities associated with tigers and other cats for instance.