So you had a lesson? What did your instructor say about your equipment? Normally if someone takes an extended amount of time to control sideslipping you can attribute it to equipment, ranging from binding setup, loose/poor fitting boots or an inappropriate snowboard.
My advice is to rent a board for a day, take a lesson and see how it goes.
Good luck, and props to you for trying to lose some weight.
All 3 instructors I worked with thought the equipement was fine. They just told me I was taking longer to progress but was still doing well. I did rent a board for a while on my third day/lesson. It was not any better, and in fact worse in some ways.
I am not "trying" to lose weight. I am losing weight. I have lost 120 pounds so far. I am feeling great, and would seriously rather eat a bullet than go back to where I was.
I have done 3 lessons already...and am feeling like until I can master the things that were taught to me at those 3 lessons I am just burning money to have someone stand around and watch me fall. Once I can actually link some turns and make it town a hill (even a small hill) without ending up on my behind or my face I plan on spending some serious cash on private lessons to help me progress.....also to show me how to get on and off an actual ski lift. The big bunny hills just have moving carpets.