I feel left out by my friends?

So basically the situation is that lately I've been busy with a lot of different stuff but it seems like my friends have all the time in the world. Normally I'd still be happy for them and stuff but idk, when I hear about all the fun they're having and the different things they do I just feel jealous and left out. I know I shouldn't feel this way and I definitely don't want to feel this way but I do. I still get to do fun stuff sometimes but it's usually without them. Usually the best solution in these situations is to just the other person/persons know how you feel, but I don't want to come across as insecure or anything. I just wish I didn't feel so down.