If I move the robber in Catan to a hex where I also have an adjacent settlement or city on, can I choose to steal from myself instead of another player or if I am the only valid player?
No, you may not.
According to the official rules on the website:
(1) You must move the robber immediately to the number token of any other terrain hex or to the desert hex.
(2) Then you steal 1 (random) Resource Card from an opponent who has a settlement or city adjacent to the target terrain hex. The player who is robbed holds his Resource Card hand face down. You then take 1 at random. If the target hex is adjacent to 2 or more players’ settlements or cities, you choose which one you want to rob.
One could argue the second part "if ...adjacent to 2 players'", but I find that interpretation completely mis-aligns with the unambiguous previous rule that you do not and would not rob yourself.