Need Advice.

I've 33/F been in a relationship with 31/M. It has been a time of profound growth for me as a person. I've been supported emotionally and thrived because of it. I found a partner whose compatibility with me is stellar. About a week ago our harmonious relationship got rocky when he adopted a pet. Despite it being a loving home the event has spilled over and somehow caused him to spiral out into extreme mindsets of stress. He's extremely busy with work but usually regulates his emotions immaculately. He's under so much stress he has become irritable. His ex girlfriend was cruel towards him, and despite my loyalty he is showing signs of fear of cheating. I am all into it, and trying to talk it out but experiencing adverse effects. Best romantic, stable, healthy, intimate/sexual, experience I have had to date, so I'm trying to focus on building a future with him. Last night he told me he doesn't trust that I won't cheat on him so I'm left in a place where I'm reaching my hand out for him mentally and I fear his hand will come out of the darkness to reach back for mine. Things are back in motion and I feel this new distance that is discomforting. I'm going to go show myself some love and patience as I get through this but I would love some advice.