There are two things in your favour here, the first is that you have noticed your relationship is being affected, and the second is that you want to do something about it. This means there’s a good chance of you having a life outside work. Many people in your position do not realise how unbalanced things have become until it’s too late to restore.
The solution is incredibly simple. Make the decision that work has to stay within certain boundaries and stick to it. Set yourself easy targets to start with, like having one night a week and every alternate weekend as completely work-free zones. As you discover that doing this does not bring the world crashing down on your head, it will provide you with the confidence to extend the time you allow yourself. It is of course a good idea to discuss this with your partner first, just to check they do want you to be around at the times you are deciding should be devoted to them. There is a chance that your being busy all the time may have led them to arrange other diversions and commitments.
Having things you want to devote time to outside work can make you a much better time manager. Setting limits for work and study-related activities does not mean doing less. It should provide motivation for cutting out all those activities that waste your time. Many problems in this area are self-imposed and can be solved by better organisation and finding effective ways to say ‘no’ and ‘go away I’m busy.’