I just read through this whole post, Beginninglove, and I have one question for you:
If Alex broke up with you, would you be relieved?
If the answer is yes, you have to break up with her.
I hate to bring negative or harsh advice to the table when you are going through so much, but my reaction from your very first post in this thread was that the problem here is your relationship with Alex.
Not because Alex is controlling, but because you describe Sam as "super-hot and amazing and the sex with her is fantastic." And you don't say anything like that about Alex. Actually, you don't really describe any reasons why you like or love Alex, other than the familiarity and security of being with someone for 5 years.
Doesn't Alex deserve someone who think's SHE'S super-hot and amazing and great in bed?
I know you said you and Alex just aren't sexually compatible. I have heard of couples who have a healthy, primary relationship with each other despite not being totally sexually compatible, but that only works when both partners date other people and/or feel that sex is not all that important to themselves or their emotional bond.
Poly/mono relationships seem to work best when the poly one feels that her mono partner is just as awesome and wonderful and energizing as her other loves. In short, when she is still truly in love with her mono partner.
(Actually, that has to be true regardless of the mono/poly dynamic.)
It also sounds like Alex simply isn't comfortable with having a partner who is poly and/or dates other people. She's definitely tried to accept it, but she clearly resents you for it.
Hence, her comment accusing you of being a sex addict. (Your response, by the way, should have been to talk to Alex about how, in fact, you have started having real feelings for Sam, and that's why you want to see her so much. Not that I blame you at all for being unable to be honest about that. Alex sounds like a really difficult person to communicate with. I don't like, for example, how she uses her mono friends to get unhelpful advice which she then throws in your face).
Anyway, I guess what I'm trying to say here is what a few others have said: when you were planning to leave Alex and wanted to enjoy being single, you sounded happy and energized, and that sounded like the right decision to me.
I confess I was disappointed to hear that you are trying to work things out with Alex--simply because it sounds like things can't be worked out. It sounds like you two are just unsuited to each other, and that you would both be happier with other people eventually.
But first, you're going to have to be willing to hurt both Alex and yourself by doing the hard thing and breaking up.
I'm really sorry. Especially with the breast cancer issue--I'm so terribly sorry.
I'm not sure you should stay with someone out of fear, though. And even without Alex, you will not be alone in whatever you have to go through--you will be able to find people who will support you, whether friends or lovers or family. (Regardless of what happens with Sam--I don't think the important issue here is Sam at all).
You sound like an amazing person, and I wish you the best. But I don't think you will ever get to be yourself if you stay with Alex. And I don't think Alex would ultimately be happy either.
Don't let guilt keep you from doing what feels right to you. You deserve to be able to do what makes you happy without begging for permission to do it. (And Alex deserves someone who doesn't constantly beg her for permission to see other people when that's clearly not what she wants).
Again, I'm so sorry. I've been through something similar. I lost my best friend and all our mutual friends, and I have to live with the knowledge that they all think I'm not that great a person because I'm just not monogamous--but in the end, I get to stay myself.