Some astronaut is in space, and his spaceship is destroyed. For some reason, this dude is wearing a semi-sentient suit that keeps him alive indefinitely, and he's just floating through space for millennia.
Not all details match, but possibly Iain M. Banks' "Descendant". From this review:
The protagonist is the sole survivor of a military attack on an orbiting base, unless you count the sentient environmental suit he wears and which manages to convey him to the surface of a nearby planet. Both man and machine are damaged, but they set out on an extended trek toward a ground base