Hi Op, so I’m going to answer having been that 15 year old child in the past. Dad left mum after a few fairly absent years where he was clearly unhappy, when I was about 12. He moved abroad but was in regular contact and we went to visit a few times. By 15 I decided I wanted no contact, as I was so disappointed with how he handled it all. Nothing abusive at all, but he failed to tell us himself he was leaving, left it to my poor devastated mum, then never actually spoke about what had happened, just attempted to get on with our relationship with no offering me chances to speak about how I felt, or help me with the situation emotionally.
The thing is though, saying I didn’t want contact was a 15 year old way to express this - I couldn’t actually tell him what I needed. Really what I wanted was him to be better. Ultimately, I believed that he didn’t love me - a child/ young person way of thinking, I believed that if he had loved me enough, he would have stayed. And his failure to support me emotionally was further proof he didn’t love me. I was so hurt and angry and needed help to deal with it. Eventually he stopped making contact as he was ‘respecting my wishes’.
Now as an adult I understand that he had his own issues and why he wasn’t very emotionally literate. I also now work therapeutically with children and young people, and on this and a personal basis, I would urge you to keep showing child that you care. Your invites out will definitely not be ignored, maybe not replied to, but this is you demonstrating your love and that you are adult enough to hold and contain child’s distress. 15 is an age where so much is going on emotionally anyway. I don’t suggest though that you necessarily do everything they want in terms of money requests- just give what you can afford, explaining you are skint, and keep finding ways to let them know regularly that they are in your thoughts.
I eventually decided to meet my dad again when I was 30, and now late 30s we have an ok relationship, but you can’t hide the fact that so many years were missed, it’s not like a normal close family relationship sadly.
Sorry that’s probably too personal and not all relevant to you but I hope some of it will resonate!