What do I do? PTSD from cheating..

I have ptsd from my partner cheating… he hasn’t cheated since then, first & last (almost 4 years ago) but I still get dreams, sometimes think of it & get all the feelings back of when I first found out (heart drop, heavy breathing, etc..).. we are good together & I trust him. It’s just me. He doesn’t know I feel this, we spent 2 years talking through it & him gaining my trust again.. I know I need help, I know I need therapy but I can’t afford it. We can’t afford it. How can I get past this without creating extra expenses? He even wants to get into couples therapy ..