I am a beginner too, but I think I see why there is confusion here. There seem to be two overlapping areas of confusion, so let me try to help in a simple way...
In English, "My door broke" and "My door has broken" both mean the same thing..... "broke" and "has broken" are both past tense verb forms, even though they have words identical to the adjectives "broke" and "broken" .
In Russian from English duolingo lessons we are first taught only that the "У меня" part is used in constructions which mean "I have <something>"......In fact the "У меня" part can also mean ownership in a different way - it can simply mean "My" and can therefore be used instead of "моя".
"У меня сломалась дверь." and "Моя дверь сломалась." therefore both mean the same thing "My door broke/My door has broken", and they both use the past tense feminine verb form "сломалась".
https://en.wiktionary.org/wiki/сломалась
I know that I will have been stating the obvious in parts, but I hope it helps somebody anyway.