I don't want to boast, but
A set phrase used to preface a statement that might sound boastful, often to soften the impact or show modesty.
I don't want to boast, but I'm good at cooking.
I don't mean to brag, but he's the fastest runner in the class.
From 自慢 (boast) + じゃない (is not) + が (but), literally 'it's not a boast, but'.