Part of the issue is that you have set the relationship with your SO up as a situation where your see each other "all the time". One thing you could do is put the secondary relationship on hold all together and re-structure your relationship with your SO so that you are not together all the time. Let that relationship get used to having gaps and time apart and get comfortable with that.
Then when the original relationship is working well with gaps and time apart, you can introduce a 2nd relationship into your life.
As far as the intensity goes. I have an incredibly intense secondary relationship. But I don't have to go to my SO and swoon about it. It is not necessary for me to demonstrate how intense my secondary relationship is to my SO. My SO likes the guy and he approves of our relationship. I don't have to roll my eyes in the back of my head and say "oh my GOD- this is so intense" to my SO. I can simply say "I had a really good time with Lee last night."
Also- my SO has a girlfriend. I can tell when he has had an intense night with her. He can actually tell me more because I am okay with him having in intense relationship with another women.
The point is, we have adjusted our relationship to have time away from each other without feeling like there is something missing in our relationship AND we have adjusted how much we share with each other accordingly to our individual comfort levels.
Hope this helps!
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.