Why do I suck at cello?

Today I found myself crying listening to cello music, not because it was beautiful (although it was), but because I don't think I'll ever play cello well. I started a year ago (although took a a 5 month break, had a shitty second teacher, and am currently on my third teacher). I can't play in tune (although i at least know i'm out of tune). I've been trying to play la cinquantine for at least 3 months and I still sound like a dying cat when I play it. Usually when I'm not good at something, I put in extra time and effort and see good results. This is not happening with cello. I don't plan on giving up, but is this kind of slow progress normal for non fretted instruments or am I extraordinarily ungifted? My teach says I have a good ear (although I think he's just saying that so I don't feel bad about him being picky with my tone). I'm 18 btw if that matters.