[21] Sort of a problem here

Hello, everyone! I'm Luiz. I'm a 21yo brazilian freelance graphic designer, musician and advertiser.
I don't know if it's really a self-esteem/confidence issue, but I've been reviewing my actions and decisions and I realized I mostly don't take decisions for myself, but to please other people. It's been like this with all my romantic relationships and with people I know in general.

I'd describe myself as a very accomodated person. I rarely go out of my comfort zone to do new stuff, unless pulled by other people I trust. The most recent decision I took that I regretted was a singing course with my girlfriend. We both sing professionally already but the course would have a different approach and she'd like to try it out. So she invited me. We had already argued several times that week so I guess I accepted it so she would see I care about what she sants, but at the same time I wasn't honest with myself. I didn't want to go. I'm not familiar with the musical genres of the course, I don't know any of the members, I don't like exposing myself (even being an artist myself), the course was expensive. Once I said I wanted to go she went through a lot to get me inside, because they got a high demand of people. She paid my part in advance, asked for them to save a place for me and I was even taking someone else's place, I mean, someone who'd really enjoy. At the same time, the consequences for me denying to go would not be cool.

I didn't have any interest in learning the songs or to get familiar with the genres to prepare myself for the course so I called my girlfriend and decided to be honest with her, saying I wasn't feeling comfortable with this course and of course saying how sorry I was for accepting something too early and mostly to please her, and also sorry about what she went throught to get me a place there. At first she was mad but she thanked me for being honest.

Anyways in the end I decided to go to this course because if anything (if I hate the place and everyone) I'll always have her by my side. But I don't know how much time with a person like me can one stay.

Thing is, if I was to be honest to myself everytime someone invited me to do something I wouldn't do 90% of things and would be seen as a boring or uninteresting person, but on the other hand I would be honest to myself everytime. However, if I was to accept most of the things to please the other person and to maintain a healthy relationship (not romantically speaking) I could not like the outcome of the decision and would be very frustrated with myself ("why did I leave home tonight?" sort of thing).

I have no idea how to get better at this. Any help would be amazing. Thank you all!