I'm so sorry you had such unhappy rides on your mtb, but I'm glad that you have a great saddle on your roadie and that you're enjoying that one.

I think Kiwi Stoker's points are all good ones. I'd be tempted to hold fire on the cyclo cross bike, keep the mtb for a few more months and try it again in the spring, if fitness permits. Then decide if you want to keep it or replace it with something else.