Wow you have been through a lot OP
From what I see from the posts that are written, you and your partner who you are not married to have a small child, have recently gone through a miscarriage and are still grieving for that child?
You have been open with your partner about these feelings, but is it right that he is almost not listening to them saying to work on the relationship. Your relationship was over and then you fell pregnant with your first child so have tried to make it work for your baby?
Firstly, as I am sure you know, a baby will make the relationship harder not easier.
It also seems like you are at different points in recovering from the loss which is also going to put a strain on it.
If you didn't fall pregnant with your first child would you still be together?
This feeling that you love him but are not in love with him, which is also about the absence of another kind of feeling, as a sign the relationship is not going to last. Although this may prove to be the truth, there is also a chance it isn't, especially with all the positives you have for him - he is amazing, your best friend, you don't argue, as parents you offset each other quite well.
Some days are cloudy, some are stormy, some are gray, and sometimes the sun shines. Relationships are seasonal and cyclical, and the statement, "I’m not in love with my boyfriend" can mean many more things than "it's time to leave." Yet it seems that for so many people, that feeling is interpreted as the end of the relationship—or at least Very Big Trouble.
It depends on are you saying: I want out of the relationship and am clear it's done, and I want to be nice about it. I don't want to hurt my partner's feelings, and this is easier to say than "It's over."
Or: I've met someone else with whom I feel alive, like I used to with my current partner.
Maybe: The person feeling this is depressed and, since the Technicolor has gone out of many things they once enjoyed, this has happened in the relationship as well. Sex can be rekindled, intimacy can be rediscovered, and depression can be treated. Relationships need constant nurturing, love, energy and input. Time management with working lives and managing children and finding quality time for a couple has to be in place.
It is a difficult decision to leave a long-term relationship, but it's even more unfair to compare the newness of a romantic love to the familiarity and comfort of a long-term relationship. Perhaps there is a depth of love that has become likened to wearing slippers and become familiar? Is this perhaps the real love? Could it be that the initial head over heels is in fact little more than sexual lust and desire, a sparkle in the honeymoon period?
Regarding your original question, if the relationship is truly over, and you don't want to work at it, then it is fairer on your partner to leave sooner rather than later. I would certainly not consider having a child in such circumstances.