Welcome home to the Church!
Just being overweight is not a sin of any kind. There are many reasons why a person can be overweight. Overeating is only one. Medications can make a person gain weight–I had that problem until my doctor put me on another one. The sin of gluttony is a mortal one, but like all mortal sins no one can accidentally commit it. First of all the sin must be mortal, you must know that is it mortal and you must commit it with full knowledge and full will. Most people who overeat do so as a habit, like chewing one’s fingernails. This takes behavioral correction, but it’s not a mortal sin. The fact that you are endeavoring to curb your eating and be healthy is evidence alone that you are not guilty of the sin of gluttony. Gluttony is overeating or overindulging when you know it’s wrong but you deliberately do it anyway regardless of the consequences. There are people who do that, I’m sure, but you don’t sound like one of them to me.