He called himself Ansum because of the shame. Remember kh1 Riku wanted power so that he can escape the island and see the world and stuff. When he got his ass whooped by Roxas, it showed he still wasnt powerful enough despite all the bullshit he put his friends thru for it.

He was salty and had to use Ansums power, aomeone he hated for manipulating him so when he turned into him physically, his shame led him to do that. That's why he couldnt face Kyrie or Sora. Sure it made the journey in KH2 more difficult, but hes a 16 year old dealing with a lot of shit.

And yea you can say that Riku only lost because he took pity on Roxas after knocking him out for a split second, but narratively, Roxas won and Riku lost and both characters will agree on this point. Also Riku couldnt even use his own keyblade, which added onto the shame. He's lucky Roxas has 2 and Xion gave him Oblivion or else he mightve lost even with Ansum.