Please don't feel broken. There is absolutely no need to. Plenty of people go through their lives being monogamous with nothing awful happening to them.
Here is my take on your points:
1. It is unrealistic to think one person can fill every need -
Of course it is. But being monogamous doesn't mean that one person must fill every need. I am currently in a monogamous relationship and my life is filled with love. I have a wide circle of friends and family that I keep in close contact with.
My partner doesn't even meet all my sexual needs. I have regular, excellent sex with myself. I am my own best sexual partner and have been for years.
For me being monogamous means that I am not having sex with other people. That's it. Nothing about one person meeting all my needs.
2. It leads to a greater sense of fulfillment by loving more than one.
It might do and it might not do. I think that depends very much on the people involved. I would suggest reading through these boards (and maybe some on monogamous relationships too). I suspect that you'll find on both sides that some people have an enormous sense of fulfillment and others are living a nightmare.
Loving more than one is no guarantee.
Right now I am not attracted to the work that would be involved in having more than one romantic relationship. I think I would find it oppressive and annoying - that is why I have one romantic relationship at the moment.
3. Allows for healthy amounts of alone time.
Again that depends on the individuals and on their needs for alone time. What might be healthy alone time for one person would be horribly lonely for another.
Plus anybody with multiple important partners are likely to find themselves struggling for alone time.
I hope you feel better soon.