I can kind of relate to your experience in a way as a similar kind of thing caused some obstacles for my practice a while ago as well. In my case, it did pass with time and though it took a little while for me to be convinced, indeed there are better days ahead!
In general, something very helpful is to try to be the cause of happiness for someone else in some way. As I noticed too, when we are spending time thinking about someone else, we don't have so much time to think about ourselves. This can also lighten up our mood as well when we see someone else smiling or appreciating something we did
In terms of practice, I found it hard to find the motivation to practice, and when I did I became frustrated by my emotions and the way I felt. I really think we should try to practice daily no matter what the outer circumstances, no matter whether we seem happy or sad. To me this also lends a little more towards the view of reminding ourselves time and time again that the nature of samsara is suffering, and by practicing no matter what the circumstances we face it helps us to keep both a reminder of why we are practicing dharma, as well as cultivating some diligence. Also, consider for a second that at least you have the merit to be able to know, study and practice dharma and therefore have a direct means for transcending the suffering nature of samsara. Rejoice in this!
As far as actual practice goes, not sure what kind of practice or meditation you do, but I think it may be good to contemplate impermanence (or even all of the four thoughts) and the fact that our attachments to these appearances that we mistake as being permanent in nature is a big cause of our confusion. You could also try to just rest and maintain awareness of these thoughts and feelings and just treat them with equanimity. Don't try to force these thoughts away. Thoughts and emotions come and go so you could even examine that aspect too.
This sounds a bit hard maybe, but as you can never go wrong with good ol' bodhicitta, maybe even recite some prayers or dedicate merit to your girlfriend, and all others who are suffering in a similar way too. Identifying the suffering of others with your own suffering is a great way to begin to cultivate bodhicitta mind.
And just go with the flow!