So you have needs he is not meeting. You would like to make him aware of that.
'Sometimes I feel with the texting full conversations with someone else while I'm there, with my asking for reassurance after you have an elongated evening with someone (and usually not getting it) or not hearing back from you for a while after these encounters... I tend to feel secondary. Not as much a priority as these encounters can be for you'.
This does not focus on what behaviors you would like him to do INSTEAD.
Could be like this:
"When you text full conversations while I am in the room? When you have a date elsewhere and do not take time to reconnect with me even after I ask for reassure? These behaviors do not demonstrate considerateness toward me.
Please turn phone off when you are with me. Or schedule separate and not overlapping face-time and text-time dates. Please reconnect with me after a date elsewhere by greeting me, kissing me hello, asking me about my day. This is considerate behavior toward me. Those types of behaviors would treat me how I want to be treated as your primary. Could you please treat me this way?"
Could keep it to the behavior done/not done and focus on what you want more of.
Not leave the focus on what you do not like.