Why do I keep getting annoyed at my boyfriend, and how do I overcome it?

So here's a little back story. We have been dating or almost six months. Our whole relationship has been a rollercoaster and it generally has brought us a lot closer. However recently, I have just been getting really annoyed ta my boyfriend over minor reasons. I know you might suggest that I am falling out of love and what not, but it isn't like that. I have a lot of personal issues going on that might be a causing factor as well. And we have had a few fights recently. When fixing the issue, I generally will become calm after the craziness, and become really naturing. But recently, I have been so cold during the arguments. Today is also another example. We were messing around and joking with one another, and he was saying some hurtful things but I let it slide. I did the "uno reverse" on what he said to me by saying it back to him. he got upset about it. I didn't say it out loud, but in my mind I was like, "He's such a baby". I have never been like that and have never invalidated someone's feelings. I feel so wrong for even thinking it, let alone my mind just keep going on like that. He is quite a few years older than me too, so maybe I'm expecting more of a mature response from him? I just want to stop being like this. I want to be more excited for him. As stated earlier I have been going through a lot, and so when I see our old photos, I get annoyed as well. I am son annoyed at him for no reason. I love him. I just want things to go back to how it was. Some advice please? I love him and DO NOT want to break up with him.
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*Update*: We both have been struggling in this relationship. Breaking up felt the right decision for the both of us at first, but continued to stay. There is a lot of work to be done into fixing this relationship. I also figured out as to why I was upset and annoyed with him so much. The way I was feeling is 100% validated. But this relationship is something we are both working on.
Why do I keep getting annoyed at my boyfriend, and how do I overcome it?
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