This is a rather thought heavy explanation as to your worth here, so bear with it.
You are a proverbial butterfly, the one whose wings cause hurricanes. It's an invisible process and subject to countless factors but it happens. Every action you make completely changes the course of history. The simple action of buying food has countless outcomes (depriving someone else of it, influencing either their attitude or their available money, supporting the store, creating carbon due to the transportation of a replacement product along with the production), all of which cause even more actions to occur. All these results invisibly alter society through emotional responses of others which in turn change their actions.
Take this thread for example. In the process of trying to cheer you up, people are sharing videos and such that other people might never have discovered. Maybe this cheers up someone else, or ends up taking up time that would be spent elsewhere. Either way, the rest of that person's day won't be the same as it would have been had they not seen that particular video. This ripple effect spreads throughout social circles then, as the person's interactions with their friend/family/colleagues are influenced by the experience (even if it's just that it wasted their time from doing something else).
It's easy to see a lot of what this chain can produce as negative, but it relies heavily on what-ifs. Say that depriving someone from purchasing something is the outcome of buying it. Sure they might be mad, but maybe that money goes towards something better for them, or they have someone else take time to calm them down, which goes on to prevent that person from being at the right place at the wrong time, or just gives them a new take on life following a slightly philosophical discussion with the angry person.
Even something as seemingly destructive as the outcome of producing more CO2 could be a seen in a different light here. According to some scientists, this would actually quicken the ice age cycle, resulting in the reformation of glaciers sooner, which would go on to drastically change the landscape until they melted, which both the meltwater and the new surface would allow for life to form in a way it couldn't before (and might not have if the glaciers didn't begin refreezing as soon as they did).
Your worth in this whole process is immeasurable. While you might be part of something bigger that doesn't really see you, remember that your action influence the whole more than anyone realizes.
Anyways, as for getting down to specifics here rather than just your worth as an individual
You've made me laugh on these forums, enough so that you're definitely one of the people I can recognize here.
While you see yourself only putting in a small amount of effort, you're going back to school for a major, and have stuck with it even after the initial reason has gone. If you truly weren't dedicated, you'd have left by now. Hell, you're trying right now to make certain that 12 people can count on you at once.
While I know nothing of your marriage, I will say there is no use dwelling on the past. Dwelling never did a thing for anyone. Instead, if you must revisit those memories, reflect instead of dwell. Find the mistakes and the solution using hindsight and shape your future action in an appropriate manner to fit what you want to be. We all have troubles, some greater than others, but what we do because of them is what defines us.
Finally, I have one piece of advice for you: release your emotions the old fashion way. Yeah, I know society says you shouldn't do it, but scream, cry, howl, whatever you need to do to get this stress out of you. Then find something funny (a comedian, a webcomic, a song, etc.) and let it balance a little bit of the stress.
And of course here's you obligatory video link, with a bit of helpful advice in it
