Clinginess

I'm sorry that this isn't a post strictly about a romantic relationship. It is however a post about one of the closest people to me - my best friend, and you have all been so supportive when I went through a breakup with my ADHD boyfriend and I have found so much solace in this forum that I want to seek some advice.

I experienced this a year ago with my former boyfriend - and now I'm experiencing this with my best friend (hyperactive ADHD). They're basically suffocating me in our relationship in the constant need for us to meet. I love spending time together and it's one of the deepest and most fulfilling friendships I've ever had, but I feel like they have the entire month planned out to the minute and every my attempt to say no is met with their huge nagging to rearrange my other plans - and disappointment. I'm also an introvert who values their alone time, and like my own space. And I'm also a self-diagnosed people pleaser... so hard to say a plain and definitive no. 

I remember an illustrative example from my relationship (and I think this is why this kind of behavior now triggers me in other people, because I had so much of it in the past). We went for a day trip with my ex boyfriend, lasting from 7 am to 11 pm, spending together every moment of the day, and at the end of it, when we came back home, it ended up in a big fight that I didn't want to stay up and party through the night. No amount of time seemed to ever be enough for him. This now seems to be the case with my friend. We have holidays planned together in a couple of days and I told them I'm unable to meet up beforehand (I'm newly self-employed so all my business depends on my diligence and keeping up with projects, sometimes I have to work at nights or on the weekend and sacrifice meeting friends those days, I do try to make it  up in quality time). They are however very disappointed and not speaking to me because of this. I mean we have almost 3 weeks together planned in a couple of days.

What do I do? It seems like it comes down to fulfilling my own needs versus their needs and I always feel bad about putting mine before theirs because they seem very disappointed in me. I don't want to lose them but I can't feel guilty all the time like I am now!

Thank you