Vary your sentence structure

"Alex was breathing so hard that his chest was burning. He didn’t dare look over his shoulder as he ran. He just kept putting one foot in front of the other and hoped he wouldn’t trip. He was still being chased by the monster."

[Subject][verb]. Each sentence follows the same formula, which is probably the main cause of pronoun overuse.

"Alex was breathing so hard that his chest was burning. Not daring to look of his shoulder, he ran. Right now it was more important to avoid tripping, so he just kept putting one foot in front of the other. Behind him, the monster still followed."