One advantage you have is, you are a fresh graduate & probably still can have time for MBA. What some of my friends in B schools tell me is, quality of work experience is as important as your GMAT, you can't ignore either of them.
For writing GMAT, if you are not feeling confident, I suggest not to give a shot at it. Since, you are telling that this is a new job & you want to make use of it, I think it's better to keep studying everyday for few hours & when you feel confident, take some time off, like 2 weeks, study & write GMAT. I guess the key is to feel confident & comfortable about GMAT.