Tell her that she can still come home for hugs and kisses and talking whenever she likes, because deep down you love her with all your heart and hope you don't lose her completely in the long term. Her happiness is supreme and you have no desire to go out with other girls.
Tell her that it is okay for her to love BOTH you and Terrell at the same time, and that she is free to date Terrell and have sex with him as often as she chooses.
Ask her if she would like you to help her buy some very sexy clothing so that she can look and feel her best when she is with Terrell. Does she want to fix her hair up special?
I think it is a good idea for Terrell to pick her up for dates at your place, and maybe he should stay the night occasionally so you can show him that you approve of him dating your girlfriend.
You could sleep in the spare room while they sleep together. This way you can see them together and feel less in the dark about what is happening.
It is important that your girlfriend sees your co-operation with her choice of lovers, because she has just had her first fuck with Terrell and her emotions are running wild about that. She needs to feel that you are GENUINE in supporting her to GROW her confidence with having regular sex with someone other than you.
Remember that you have repeatedly told her she can have sex with other guys. You must now show you were being HONEST saying that (even though you secretly were struggling with whether that was a true statement).
You must KEEP your word as an honourable man, and continue to allow her to bareback or use condoms as her right of choice.
When you talk to Terrell, you must show him proper respect because he is "adding sexual value" to your girlfriend's life and providing her with companionship and taking her out and spending money on her for social fun. She is also widening her circle of friends.
You must also say nice things about your girlfriend to her friends so that they can see you are a quality man who will take care of her and always put her happiness above your own at all times. In your mind, you place her on a pedestal and "she can do no wrong in your eyes".
Your girlfriend is worried because you are agreeing 100% with everything she has asked her. You have not said "no" to anything - this makes her feel suspicous that you might have a secret girlfriend that you may be hoping to have sex with.
I think it would be a good idea to google "Literotica" and read lots of the stories about cuckolds and hot wives, as this will help you to prepare your mind for the range of emotions that cuckolds experience.