Help!

I have been dating this guy for 6 months now, and recently I have been feeling so down and upset, and a lot of the time it is while I am with him. 
I know that parts of why I am upset are from other things going on, but I'm confused why I am always the most upset while I am around him. 
Today I was convinced I was going to break up with him, and yet I can hardly say the exact reason why. 
Has anyone else experienced this or have any ideas why I feel like this? He has been so lovely to me, being there for me while I have been so down, but I just don't understand 😪