There are several things to consider, ideally before you cast your chart.
Firstly, the question itself. This usually appears to be the easy part but often it has hidden issues that you need to consider as part of the process. The way your question is phrased is as a clear choice. To tell or not to tell. That's good and allows you to move on to the second consideration.
Which houses are involved in this horary? The question deals with your relationship with your boyfriend and so the answers are: The first house, which signifies you and the seventh house which signifies him. No other houses are relevant to this question.
The third thing to do is to consider what the possible answers are and how the chart would look for each. You can do this before or after you cast your chart but it is something that you need to think about before drawing any conclusions from the chart. There are clearly two possible answers to the question. To tell him it's over or to wait. What the chart would look like depends very much on the circumstances of the question. And that's where you come in is as the querent.
Once you have cast your chart, you should look for any 'considerations before judgement' - does the chart show any problems in reaching a judgement based on it. I'll deal with this at the end.
You know the circumstances. I can see two variants. The first is that you are not sure whether to tell him it's over because you haven't made your mind up yet, or that you prefer to wait. Telling him in this situation requires action on your part to force the decision. The default is to do nothing. A chart with no connections between significators would indicate no action. You leave things as they are. Telling him would would show up in the chart as a a connection between your significators and his.
The second possibility is that you have already made up your mind to tell him and are considering the 'wait' option simply to spare his feelings for a bit longer. Here the situation is reversed. You're looking for reasons not to tell him, and that requires a change in your plans. So no connections indicate that you tell him now, as per your current plan and only if there are connections do you thing about postponing the event till your return.
That might seem a little complicated but it isn't really. You know whether you have already made up your mind to tell him and are simply considering whether you should wait as solely one of timing the deed, or whether you are not sure yet about telling him and are using the 'wait' option to give you more time to make up your mind.
So let's see what the chart does indicate. We know you are Mercury in Capricorn and we know that he is signified by the seventh house ruler, Jupiter who rules Pisces. Jupiter is in Leo and happens to be in your twelfth house. Mercury is in your fifth house, so there's no connection between the two planets. So the first testimony is to do nothing to change your plans.
Mercury is under the beams, so cannot be seen or 'see' clearly. That reflects the duality in your question. However Mercury is ahead of the Sun and moving out of the beams. Mercury in Capricorn is in the Fall of Jupiter, indicating that your feelings for him are not positive here. Jupiter is in the Terms of Mercury, so he has some feelings for you but they're not really very strong. He's definitely more enthusiastic than you.
The testimony of the planet positions by essential dignity does not bode well for the relationship. It confirms what you said in your post. Mercury is well situated by house but Jupiter is in the twelfth, you house of loss and enemies.
The Moon acts as a co-significator for you. I see that more as the role of a 'fairy godmother' or 'fate' that works in your favour, rather than signifying you personally. The Moon does trine Jupiter and it's in an applying trine at that. But it's conjunct the South Node, which is seen as 'malefic' and it's in the eighth house of death (I've use whole sign houses) in Aries. So this trine is not as positive as you might think.
You asked who or what Mars represents. Mars is a natural malefic and a square or opposition to one of the significators would work against the event coming to pass or would slow it down. Mars is in a mutually applying opposition to Jupiter. Mars is also Lord of the eighth house of Death, which here is the signification of the death of the relationship, not the death of your boyfriend.
So the evidence from the chart is that the relationship is indeed over and that last mutual application between Jupiter and Mars suggests it's over fairly soon.
If your initial feeling was to tell him now, then there's nothing in this chart to suggest you change your mind and delay the action. If your initial feeling was to wait and that is what you would prefer to do, then this chart does have some evidence for telling him sooner rather than later.
Now I mentioned 'considerations before judgement' earlier on and this chart has one. There's a late degree of Virgo rising. So you should think whether that stops you acting on the chart or rejecting or modifying the conclusions you've drawn from it (or your Astrologer has drawn from it.) Lilly argues that it's not safe to draw any conclusions from such a chart, unless the degree arising corresponds with the age of the querent. He doesn't quite explain this, as I can't believe that Lilly had no querents over the age of 29 and that he did have querents aged under three. Some modern users of horary have tried to insert other meanings, such as 'it's too late to do anything' with such a chart. No matter what you do things will take their course.
Personally I use this consideration only when I'm not quite sure what the question is and it would be dangerous to make a decision. Frawley argues that these consioderations might have been valid in Lilly's time because it was not always easy to tell the time and thus no when a sign change had taken place. So a chart that gave an early or late degree rising might actually be for the 'wrong' time and in reality another sign was rising, meaning the key sigificator was another planet. These days we do have accurate means of telling the time, so he argues we should forget the consideration entirely.
This chart does seem to describe the situation and I think it is probably safe to ignore the consideration.
