Hello Darien 😊
I dont think it is selfish to want to have some time to yourself. I have been parenting for years, and know for sure that time to oneself and balance in ones life makes one a better parent. The main thing is not whether you should take the time to yourself or not, because everybody should have a certain amount of freedom to balance with their responsibilities. The important thing is how you communicate this need to your significant others, in a way that they will receive it well, or fairly well.
The way I communicate it is to consider their needs and what they want while I am considering my own. Then it is easier to sell the idea of my travelling to them. For example, I tell my boyfriend that X amount of travel would cost the same as a computer he covets. Then I suggest we save for both simulateously. He is generally very open to these suggestions.
As for you and your life: You are currently in one of lifes most stressful and difficult situations. You are very young, and have more responsibilities than anybody should have at your age, and so does your girlfriend. Most of us have learned how to cope with lesser responsibilities, before we added kids to our lives, so it came a bit easier, even though parenting is always a challange for even the most mature and responsible adults.
Dont take your girlfriend and son, because from what I am hearing, travel is your thing, and she has not expressed a real interest in it, so therefore a waste of money and a waste of your break. You will argue. I can promise you that, if you try to push yourself beyond you stress limits, by travelling with a newborn, in an already tense relationshiop. Do however consider more the money situation. She does not work for money, but with a little baby to look after she is certainly working, and a lot harder than most who have a 9 to 5. Consider what she would like for herself, which does not include you or the baby and offer it to her. ie Share some of your money with her, because you have more freedom to earn, and she is the mother of your child. This is an important relationship, so take care of it. Maybe you will or wont stay together as a couple, but you will always be the parents of this child.
Also, does 7-8 months give me enough time to save or am I being unrealistic? I ask because since she isn't working
Make out a realistic money savings plan, and you will be able to see more clearly what you will be able to save. I have one of those, and it allows me to guess quite accurately what travels I will be able to afford within the coming 12 months.
Her mother thinks it's an impractical idea and my girlfriend just called me selfish and got mad at me.
Speaking of her mother and others who may be helping or supporting her. I think, it will help others to see your needs if you add yourself to the list of your girlfriends helpers and supporters. ie Look after the baby while she eats, or has a bath. I assure you that those things mean a lot to a new mother who gets very little time to herself, and they will score you big points in her eyes.
Dont get too upset when your girlfriend gets mad at you. Try to let it go, because she is stressed out and sleep deprived right now, which does nothing to help her ability to see your good side, or that you might have some stress to contend with too.
One other thing: Travel has its benefits and can give a person character, but it is not a magic cure. It wont make your fears and anxieties fade away, and 2 weeks away will not put the sensible and responsible head of a 40 year old on your shoulders.