The few times I've let it out in front of a girlfriend though have been the beginning of the end of the relationship. It's like they immediately lost their attraction to me when they saw me cry.

I'm very sorry to hear that. It may be that they did not have the emotional and social maturity to process it well. Or, maybe your expression did not come across in the way that you thought.

Regarding the part about feelings, what should we be doing when we understand our feelings?

This is one that I can't answer as an expert, both because I am not a mental health professional and because I struggle with my emotions a bit due to my ADHD and maladaptive coping mechanisms to deal with childhood trauma. But, therapy has helped significantly and I will always suggest it to anyone who is able to access it.

What I can offer, though, are some tools, theory, and suggestions that have been helpful for me so far:

Find a good Feelings/Emotion Wheel. So far, I like the ones patterned after the Junto Institute as it delves into the nuance of emotions that we experience.

How do you use it? Well, there are a lot of different approaches. What I find helpful is looking at it from time to time to "look at the map" and thinking about times when I have experienced intense emotions, using the Wheel to better draw out more precisely what I was feeling. This exercise generally also goes into exploring why I was feeling that way and contemplating what ways I could act in order to express the identified emotion in a manner that is both genuine and constructive (I am much more comfortable with logic than emotionality).

When it comes to interpersonal expression of one's emotions, one can try the same thing with a bit of extra roleplaying. First, I might walk through how I was feeling and how I expressed it, then pretend that I am the person who I expressed it to and try to identify how I would feel in their place and why (every other person is another human being with their own hopes, dreams, desires, and emotions).

An extremely important thing to keep in mind when working through past experiences is to be kind to your past self and past people that you interacted with. Malice is not a very common thing to encounter, so try not to assume it.

The idea, overall, is that by going through exercises like those, one builds their comfort and familiarity with their own emotions and are better able to self-regulate and express themselves in a manner that will lead to more healthy outcomes.

Going back to the first bit of yours that I quoted, if you did indeed express yourself in a healthy and appropriate fashion, splitting ways may have, in fact, been the healthiest outcome for you. Being with a partner that does not value you for who you are (our emotions are part of ourselves), is not something that is psychologically healthy or conducive to a stable relationship.