Friends have been hanging out with out me, what to do?

So since the start of summer my friends have constantly going out without me. Ive been working Monday-Friday 10-5 to save up for university and I have summer courses, so I would tell my self that they didn’t ask me to hang out because I was busy with work or school at the time. But today is BC day and they know for a fact I don’t work and that I don’t have classes but still the 6 of them hung out without even asking me. I feel like such shit and don’t really know what to do.
Some people are gonna say that I should look for new friends but I’ve known these people for 6years and their my core friend group also its hard finding new friends when all I do is go to work, to the gym and then go home to do my online courses.