What should I do about my friends?

Okay so Im having some trouble with the group of people I hang out with. I was starting online college to get my Bachelor's but online classes weren't clicking for me so I ended up switching to a local college and started studying Medical Assisting. This college program is an accelerated program so I have a lot of work to do. Around this time I started working at Walmart as well so I have had very little free time since I started school/work and I like to spend that time with my girlfriend. I am loving the medical field so much that I want to continue my education to be a Nurse Practitioner or even a Doctor. Since starting school/work, I have been wanting to settle down and become financially set and get myself set up for a great and successful future. I have been making goals for myself and I feel like Ive become more mature. My girlfriend and I both encourage each other to do well in school (shes studying to be a Physician Assistant starting next year) and we have both become a lot closer to each other this year and we both have similar goals and want to be successful. My group of friends have a problem with me being so busy and not having time to hang out with them and goof around. I am content with not hanging out with them because i am more determined on getting ahead and focusing on my relationship with my girlfriend (because I want to marry this girl in the future) than goofing around. I am 19 and they are in their twenties so I feel like they should understand my point of view. I am starting to feel guilty because they are upset that I dont have time for them and its starting to stress me out. What do I do about this situation?