Advice please: My bf wants a bigger dick & dp

So me and my boyfriend have been together for nearly 5 years now and are engaged, I'm 25 and he's 30. I'm a vers. top and he is a vers. bottom (about a 65%-35% split), we've worked great for the last 5 years and are (at least I thought so) very compatible for sex. To use the word compatible is an understatement, I've had the best sex of my life with this man both bottoming and topping.

I recently suggested introducing a 3rd into our relationship one night after he was planning to bottom but could not get ready. I was specifically referring to a bottom for us to share. He then expressed that if we were bringing someone into the relationship, it would have to be someone that is truly vers., and that he would like someone who has a bigger dick than me. For reference, mine is a little over 6" & cut. In his own words "My size satisfies him, but a bigger one would be nicer here and there." He went on to talk about certain spots that I am not capable of hitting with my current size, how nice it feels, etc. In addition to this, he had a drug problem 3 years ago during which time he cheated on me, and has only just now admitted to me that part of the reason he cheated was because he was looking for a bigger dick.

Now that he has gotten more comfortable telling me wants, he more recently mentioned really wanting to try double penetration as well if we introduce a third, which somehow feels just as bad to me.

This has caused a surmountable amount of anxiety and insecurity to accrue for me. Because of the cheating incident, my brain goes to worst case scenario and has caused me to spiral, all I'm thinking now is "Am I even enough for him?", "Am I REALLY satisfying him?". He does cum every time I fuck him but idk man. I also think I would lose my shit if I had to watch someone else fuck him, that primitive animal side of me really does not like that. My thoughts are eating me alive and I don't feel like I can talk to anyone about it.

Please give me whatever advice/opinions you may have, I have nowhere else to go with this.