DuPont - we're talking about long established patterns of interaction between you and your mom and those are hard to break. FWIW its usually best to be direct...

Confront her on the issue before you go. I don't mean start yelling "Mom I hate it when you..." (or "Mom, if you loved me you would remember/understand and not try to sabotage me like this") but just make some really clear I-statements such as "Mom, I know that you are trying to look after me by bringing me foods you know that I love, but I really need your support in helping me with meeting my goal of losing weight." Then suggest an alternative such as "Can I send you a list of foods it would be helpful to have in the house so I can stick to my new way of eating?" or "Perhaps we could go grocery shopping together when I get there?" Don't expect her to be happy about this...but as long as you are clear, unemotional and firm she's not going to have a choice but to accept that you've taken a stand.

Good luck,

Lisa