He was a wussy in the beginning, so he needed to grow some balls, even if they were parasyte balls. He was also too weak to realistically survive continuous encounters with full parasytes. If it takes a forced personality change to the point of declaring the old Shinichi dead and a new one born, then so be it. It's the price to pay to get us a hero instead of a loser who couldn't even survive.
Managing to fight with the mom-monster wasn't necessarily a stretch. Uda's parasyte also noted her attacks were without inspiration and easy to read. So, basically she was doing half of Shinichi's job for him. This and earlier fights make me think full parasytes are in fact less intelligent, in practice, than failed ones. They have it so easy that they never needed to struggle or try to compensate for anything, making them less resourceful and adaptive in battles. This was apparently Uda's parasyte's first fight (since he was delighted to see how full ones look like), yet he did remarkably well.
It'll be interesting to see if the girlfriend can accept the change in Shinichi. It's like she started to date a semishy nerd, but suddenly finds she's dating a semitough guy, haha.