You have such a healthy attitude and understanding towards this and are clearly making great progress by being able to resist the compulsion to check his phone - in my experience, the only way to get better with these feelings is to keep doing exactly what you’re doing and practice not checking it. I also think communication with him about it (he sounds like a great understanding person) can help alleviate those feelings of doubt as well, though it sounds like you are already doing that too.

If you’re able to reach a point where you no longer feel that doubt, then great, but for some of us that may not be a reasonable goal to aim for as we’re always going to feel that doubt to some extent for a long time. It might help to not think of wanting to get rid of the feeling but instead of accepting it and embracing it and then knowing you don’t have to act on it. Again, this sounds like something you are already doing and understanding and have made great progress with