If you can't fulfill the vow to be vegetarian because it is harming your health, and esp. if you did this as you say due to some kind of over enthusiasm, then eat some meat but at least keep the intention to hold the vow. Then later try to fulfill the vow as best you can and perhaps gradually become more and more permanently vegetarian (or as much as possible).
I have actually heard lamas advise people to do exactly this given this circumstance.
The vows are not intended to harm us. It is better for bodhisattvas to live in this suffering world than get sick and harm their body unintentionally.
Similar to Kirt, I think it's a good idea not to give up on the vow but to back up a tad and start over more gradually, with a view toward taking the necessary steps to being able to keep the vow straightforwardly. The way I'd start would to prostrate to the 3 Jewels, go for refuge and generate bodhicitta, confess in front of them your lack of preparedness and ability to immediately fulfill this vow you've made, vow to steadily work toward completely fulfilling it as you originally intended, and then follow through with that. I think you should begin by cutting back on how much meat you eat (such as maybe every other day, or every few days, or once a week, whatever you can healthfully manage, and only eat it for nutrition, not due to any craving if you have any), while also educating yourself on how to healthfully eat vegetarian. It's not hard at all once you have a little education. Then gradually reduce your meat consumption until you've completely eliminated it.
If you're concerned about purification, a wonderful and powerful sutra-level Mahayana purification practice is the Bodhisattva's Confession of Ethical Downfalls. It can be freely downloaded here in format ready to be put together as a practice booklet: http://www.fpmt.org/hope!/a4/booklet/co ... a4bklt.pdf
. A word of advice about confession: the key is to generate regret, NOT guilt (especially not some neurotic, nagging guilt that you continue to carry with you). And also, to have faith that the objects of refuge (in this case, the 35 confession buddhas) are actually there in front of you, whether you can currently clearly visualize them or not, is a key point. It's said in the Avatamsaka sutra, among others, that wherever you visualize and consider the Buddha to be, he will indeed be right there. So be confident about that and practice with that sense, and you'll accomplish the practice.
Best wishes on your noble endeavor!