OK, having read all your follow-up posts I think I understand the situation a bit better now!
YANBU to be upset or suspicious about a 15-year 'friendship' that he apparently deliberately concealed from you. I can completely see why that revelation has knocked you for six - whether it was an affair or not, I think it would always be a bit of a shock to find out that a partner was very close to someone for such a long time without ever mentioning it. If it was affair, obviously that's devastating. And if it was just a close friendship, I think it would still make most people wonder why on earth they'd hidden it like that - even if your partner had been spending that time with another man, purely as friends, it would still be pretty weird and vaguely unsettling to suddenly find out that your husband had a best mate you knew nothing about.
However, looking at everything else you've said about your partner, I do think that this latest discovery is actually just a small part of an unhealthy and controlling relationship. It's not remotely normal or OK for someone with whom you live and share a child to buy a house in Spain without telling you and then to expect you to pay for your shared family home because he'd run out of money. Although you co-own the house he currently lives in, you've basically been paying for it. He has been 'retired' pretty much the whole time you've together, which is a bit odd in itself if you were still working full time.
When you moved out, it obviously wasn't a clean split, as although you stopped living together you were still 'loving' towards one another and he didn't seem to want to separate, just to have you as a girlfriend who lives in a different house - but he also didn't want to sell the house you own together. He didn't support you through your difficulties with your parents' dementia, and just left you to deal with it on your own. Basically, he's been calling all the shots for the whole relationship and has been gaslighting you into believe that somehow you are the problem. You are not the problem. He is the problem. He honestly sounds like he has some kind of personality disorder to me.
He has treated you very badly and you are doing the right thing to get a solicitor involved. I also wonder whether you might benefit from some counselling to talk all this stuff through? It might just help you see things a bit more clearly through the fog of emotions you're experiencing right now.
All the best, OP - you deserve better than this man.