。
I can't believe it myself.
Literal
Self [even-でも] cannot-believe-[potential-negative-られない].
Short, common expression of self-directed surprise. 信じられない is the potential-negative form of 信じる ('to believe') — 'cannot believe.' Used for things that defy one's own expectations. 自分でも ('even to myself') adds the note that even the speaker themselves finds it hard to believe — implying the fact is astonishing even to the person most likely to know it.