boba fett couldn't retrieve his own helmet and armor in 5 years on the same planet ?

It's like this. He couldn't get up and get his armor back until he was fully recovered from being swallowed by the sarlac, and it took five years.

First there was the hospital stay and extended rehab, which took a lot longer than it should have because the medical system on Tattooine is crap. And then there was the problem of getting good artificial limbs to replace the extremities he'd lost, because after the Emperor destroyed the Jedi, the prosthetic limb industry collapsed because nobody was fighting with light sabers any more! I mean, that's the sure win in light saber duels, right? Cut off a limb or two? Yeah, without that, nobody was making prosthetics good enough to be reliable in the world of scum and villainy...

For all we know, he just couldn’t find his armor or assumed it had been destroyed.

Nobody messes with Raylan Givens ...