Problams with my boyfriend making me cum

Yes, to some I may be considered young. I'm 18 now and just graduated high school in May. My boyfriend is 19 soon to be 20, and just finished his first year of college. I have been with my current boyfriend for 2 years. Yes, we have had sex many times (don't worry, we always use protection). There has never been a problem with me being turned on when I'm around him, because I always am. Up until about a month ago, he never had trouble making me cum, but for the past month, he hasn't been able to. I've tried relaxing and I can make myself cum (although what's the fun in that?), but for some reason he isn't making me cum anymore. I'm always turned on and wet and what not, and when he fingers me and does oral, it feels amazing and I enjoy it so much, but for some reason I just can't cum anymore. I've cum twice in the past month by ways of my boyfriend, but that was during sex and took me way longer than it used to. I can't figure out why I'm just not climaxing anymore. I know it's not his fault because he does and amazing job making me feel great though fingering, oral, and sex, but he doesn't believe me when I tell him that because I don't cum like I used to so he thinks I'm lying. I feel bad for him because I can see that it is making him depressed about it and he gets frustrated with me easier now, but I don’t know how to change it. I really want my boyfriend to make me cum the way he used to.. some one, please help me out and give me some advice.
btw, its not a communication error because we have tried talking about this and I tell him and he knows what makes me feel good (which isn't the issue) I just physically cant climax anymore.

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