、。
As long as you don't overeat, I'm sure you'll definitely lose weight.
Literal
Eat-too-much only don't-do-[if-なければ], definitely lose-weight [quotative-と] think.
さえ is a focus particle meaning 'even, only' — in negative/conditional contexts it produces 'as long as just X' constructions. さえ+なければ = 'as long as not X.' 食べすぎさえしなければ = 'as long as you don't overeat,' where さえ singles out 食べすぎ as the only condition that matters. 痩せる ('to lose weight / become thin') is the verb; ~と思います is 'I think that.' A common diet truism phrased with the さえ pattern.