Greetings @Mia.1,
You’ve got some great questions here! 
I really had to put on my thinking cap to ponder through these ones. As always, these are just my own thoughts, but hopefully they might be able to spark something that helps you!
When it comes to being able to send out more energy than one has, I would say no. You have the energy you have and when you use it all up you’re at 0- I don’t think it’s possible to dip into “negative” amounts of magick.
This is mostly because I see magick not as an equation in a math problem, but as an somewhat tangible thing- although it often doesn’t have a form, energy has substance in a sense. You can’t have negative apples or negative herbs in the garden. I think of energy is part of nature- it’s there or it’s not there.
(Perhaps there’s the possibility of “borrowing”- in the sense that you owe a debt and it must be repaid- energy from a deity, but I don’t work with deities in that way so I can’t attest either way on that)
That being said, you can absolutely cast until you’re running on empty- and that is an exhausting experience that often takes time to recover from. I recommend checking the guide on Spellwork Exhaustion for more info as well as how to recover.
Additionally, (and some may disagree with me on this) just because you consciously cast a spell and wish for a deep or powerful bond to break, there may be some deeper part of you that isn’t ready or willing to let go yet. This is especially true in emotionally-charged cases or things that run deep- even if your conscious mind is 100% on board with throwing a crappy person or situation out, there may be something deeper that hangs on. (This can be for a huge number of reasons- perhaps it would feel lost without a driving goal to focus on, or the energy or revenge, or simply the good memories attached, etc.)
This is why things like Shadow Work and meditations that deal with the deeper parts of the consciousness are so worthwhile (very difficult, but very worthwhile). They can reveal parts, places, emotions, and bonds that we had no idea even existed before they started interfering in some way.
Spells are very powerful things. If the spells you cast aren’t having the intended effect, it may be because there is something interfering- which could be anything from something in your deeper conscious struggling to let go, to the recipient having levels of protection in place that negate or deflect the spell, to the spell simply needing more time to come into effect 
My best advice is to continue to pursue the spells that call to you while be extra observant about them. Keep a journal- record the spell experience, how it makes you feel in the moment, and how you feel about it hours or days afterward 
. Take the time to do Shadow Work to peer deep inside 
. Be open to other types of spells or methods that call to you 
Again, these are just my thoughts and suggestions- whatever you choose to do, Mia, I hope you’re able to get past this block! Wishing you all the best with it 
Many blessings! 