I want to spend alone time with my boyfriend, but he always invites his best friend.

I've (19F) been dating my (20M) boyfriend for almost two years now. He's the best guy I could have ever asked for and he makes me tremendously happy. We rarely see each other during weekdays, since both of us are busy and live with our parents. The only time we can see each other is on weekends. Sadly I've been very busy with my studies, so we haven't been able to spend as much quality time together as we used to (almost none).

My boyfriend is very invested with his best friend (24F), since she's going through a very difficult situation. He is very worried so he wants to help her. As a result, he always asks me that we should spend time with her on the weekends. They also hang out together regularly. I'm okay with him spending time with his friend, and I'm happy that he wants to help her feel better.

This week, I finally finished my exams. The first thing I did was asking him to spend the day together, but he immediately wanted to invite his friend. At first I felt angry. I didn't understand why he wanted to spend so much time with her (since they've already hang out to play some videogames together a couple days ago). This is not the first time this has happened

Despite this, I couldn't bring myself to say no, since I've already said no on different occasions.

I really don't want to sound jealous, but I'm getting tired of this situation. On one hand I feel sorry for his friend and I want to help as well. On the other hand,I secretly wish he wanted to spend alone time with me without asking to invite her.

Now is too late to tell him not to invite her, since he already did.

Am I being selfish?

P.S: sorry for any grammar mistakes. English is not my first language.

TL;DR: Boyfriend always asks to invite his best friend whenever I want to spend alone time with him.