Hey there, welcome.
First of all: sorry you are in between a rock and a hard place. I love my partner and want to be his 'one and only' and I love my partner and want to love him for who he is … This is your side of the deal. The problem here is: Your way of loving him is making demands of the way he should love you. And I don't intend to be mean, but this concept can't work. I am in a relationship with two men, who are/live monogamously with me. Both of them. You may ask how this can work. Maybe it will help to hear, what we talked about, when things began.
I myself have been quite mono-amorous in the beginning. I was married and I was happy. But I was just seeing myself as 'mono' as long as no other love interest came along. Eventually I met my (at that time) best friend. And I fell in love. And I struggled to combine the way I regarded love and myself with this paradox new happening: Being in love with my husband, kind of waiting for this love to decline, because the love for my friend was growing. It never happened.
When I tried to explain what was going on in my head, I had a hard time. I couldn't really understand myself. I just knew, that I loved both of them. My husband asked me the same questions you have just voiced. He was afraid that I could leave him, that he would become unimportant, that I would fall out of love with him. The only thing I could do to assure him, was to show to him that nothing changed. That I stilled loved him the way I did before. It was no easy task but it worked for him.
What both of them understood, was the sentiment about what your partner said.
He said he would be himself, he would be true to himself - so what does that mean?
This means, that he can't change the way he is. This means that he can't become someone living up to the expectations of another person. He would be miserable if he became just like you would want him to be. Question for you: Would you still love him if the person he is changed because of you? What exactly would it be you 'loved' there? Your version of him? Would that be 'him' at all?
Both, my husband and my best friend, immediately said, that even if they didn't understand how this may work for me emotionally wise, that 'it was just who I am'. Because I am me. I can't be someone else and I am me with all my spleens and quirks and whatever good or bad trait there may is to be found. And I am me with my capacity to love more than one. To lose one of my loves would be the same as for you loosing your 'one and only' love. It would hurt, there wouldn't be a scale, were you can measure how much it hurt more or less than for a person loving only one.
As you have already been told: love isn't finite. You yourself have so much feelings to spend, you never run 'out' of them. For some this includes love. And you would never choose how much feelings are appropriate. 'I can't help myself, I am in love', 'I feel so much joy, it's overflowing', 'I am outraged, I hardly know what I am doing' – emotions seldom have a 'this is the right amount' scale. If you hurt, you haven't chosen to hurt if you feel joy you haven't chosen to feel it. If he falls in love he won't have had a say in it generally. I am convinced that you can't choose to feel love. You just feel it. He could choose to act on it or not. But you can never dictate how he feels.
Things that are measurable are time, money, resources of any kind. Those thing would become important as soon as there is a new person in his life. But this isn't the case right now. I don't know if this helped. Those are the basics we discovered when things got started. I hope you will be able to get a hold on this concept and are able to wrap your mind around it a bit more. I promise, it will bring you even closer to your partner