This one may be a short one, but I think it's still a very important question.
My simple answer? Get a little selfish. You don't have to be everywhere and do everything for everyone all the time. If you can't make a birthday party because you need to meet a deadline at work or you just need some "me" time, do what you need to do so you aren't a ball of stress when you stretch yourself too thin. People will understand, especially if they are your friends and family. Then, when you do have space to breath, make sure you're reaching out to people to let them know you value them and appreciate them. That will help a ton if you may feel guilty for not saying, "yes" to everything. I've heard The Best Yes by Lysa TerKeurst is a great book and it seems like it'd be relatable here. I'm working through a few right now but it's on my list of audiobooks.
In short, don't be afraid to say no, be intentional about the time you do spend with friends & family and take care of yourself!