I find an instructor to be invaluable when you're first starting out with... well, anything. Yoga, like any other work out, is all about proper form. I think it would be beneficial for you to first get someone to show you how to do the poses properly and correct you where you need to be corrected. Especially since it's such a subtle art. Little things like pushing your chest out a bit more, or tucking in your pelvis or even the direction of where you look (sounds silly but its true) can make an enormous difference. And you don't really notice that kind of stuff at first, nor are you able to 'check' yourself quite right, even in front of a mirror. An instructor can definitely help you get the most out of your experience.
As for your college dilemma, my dear boy, I don't want to discourage you from following your dream in anthropology, if that is truly your dream, but it sounds like a part of you is holding onto pursuing engineering as well. And if math is the only thing that is stopping you, well, I don't want to sound like a preachy parent, but I do encourage you to re-consider this decision. I know mathematics is no cakewalk in university. Hell, that's the very reason I pursued a bachelor's degree in English literature as opposed to the sciences or finance, and I paid for it dearly. Liberal art students really do come at a dime a dozen; I wanted to be an exception. It turned out I wasn't. Guess what I'm back in school for? Commerce. And I've got parabolas coming out the frickin' wazoo.
If engineering is truly your dream, then you will find a way to get around the maths. You are a very bright guy, I can tell. It might come a lot more difficult to you at first, since you might not be accustomed to that type of thinking, but once you get the hang of it, you really do find that it broadens your thinking because you're being challenge to pursue something completely out of your element. I have full confidence that you really could push through if you wanted to.
And never be afraid to fall behind a year. Everyone goes at their own pace. It isn't a race. People change their minds and switch out of programs all the time. College is the time to find yourself. A semester or even one academic is nothing compared to the rest of your life.
But then again, if anthropology truly is your first love, and you're willing to pour blood, sweat and tears into this as your career, then by all means, this is what you should stick to.
In the meantime, I'm happy to hear that you've given yourself some time to play and enjoy the college experience. It's sweet that you have a girlfriend. Mind you, I'm surprised you haven't had all the ladies fawning over you; she should watch out!