To dwell on something means to think about something for a long time. Agonize over means to worry/stress about something.

Agonize over means struggle, like when you have to make a decision and it's really hard. And dwell on is when you talk a lot or think too much about something bad or something that's in the past. Putting it in a phrase so you can see de difference: I am agonizing over this thing I dwelt on yesterday. Hope you got it!!

To "agonize over" something usually implies that the event is a future event and that some choice must still be made. To "dwell on" something usually implies that the event is a past event and/or that no choices will now change that event. Examples: "Maria agonized over cutting people from the guest list for her wedding, since she was limited to inviting only 120 people." (Maria's wedding is in the future. She needs to make some difficult choices about that future event.) "It was dinner time, yet Paul was still dwelling on the rude driving who had cut him off in morning traffic." It is possible to say that someone is dwelling on some future event; but it would still mean that the person had not choices about or control over the future event. And it would still imply that the information the person was thinking about was past information (even if it is information about a future event). For instance, suppose that a woman goes to her doctor on Friday, because she has not been feeling well for several days. The doctor orders several medical tests. It is Friday. He tells the woman that he will not know the results of the test until Monday. The woman may worry all weekend about what the test results will be. Her friend may say, "Don't dwell on it." In this example, the news from the doctor is a future event. However, the sickness and tests, which are causing the worry, are past events; and for now, the woman has no choices to make and no control over what the results will be. So we can say she is "dwelling on it" if she is thinking about it too much. We would not say she is "agonizing over it," because she does not have any choices to make about the situation right now. All she can do is wait.