I think it's just too rampant for a tub and a restricted place. My initial thought was to cut it to ground level and let it regrow, but it won't flower on the new growth until the next year.
So,on second thoughts, I'd have it out and, if you still want a clematis there, get one that is cut back early spring and flowers on its new growth. There's a big choice and they've larger flowers so are more spectacular.
Having said that, I love a montana in the right place, but you have to keep at them to keep them under control (I have one that's about 15 ft up a tree, it just needs something to grab and it's off..)
Do you have some sort of frame for it?