Well you cant make more hours per day. Those that manage to speend more time with a gf/bf while not taking time from something else are those that share their work time with their partner.
Traditionally humans have lived in close communities where they have worked together, their survival have been dependent on that. You and your partner where both part of the same community and your work where therefore shared.
Families (the primary group) where part of the secondary group (the community), and work was either part of the primary or secondary group agenda. Both in hunter-gatherer times and in farming times work where centered around a common theme: Get food, build protection etc.
Because of this overlap in family time and work time humans where able to do more of both than you do in your situation, and its natural to assume that this have been going on for so long that we have it encoded in our genes, and that our brains are adapted for that way of living no matter if we do live that way or not.
Now in modern society members of primary groups are members of different secondary groups or work independently on different things. Assuming that your brain are hardwired to try to have time with your gf while NOT sacrificing work time, its easy to figure out that it does not add up...
In other words either find projects you can do with your girlfriend, or find a girlfriend that you can do your projects with. Unless you make gf time and work time overlap you will have to sacrifice one.
As a result of this there are plenty of evidence that relationships that start at work are more likely to succeed. Here are one source.
If you want the current relationship to last my suggestion are that you find a way to create a common workplace so that you can share that time with her. Or you will grow more and more dissatisfied and eventually end the relationship yourself. That either directly or indirectly by showing an increasing amount of resentment towards her (in other words you will start behaving like an asshole until she leaves you).