This struggle is so relatable.
Our daughter was fed breast milk daily, and 2-3 times a week she had expressed milk from a bottle. We started with the bottle from week 2 and it went well right away! Super, we thought, she’s got this. We kept giving it 2 to 3 times a week when it suited us, like with visits or outings. She also spent a day regularly with the babysitter during these months and drank from the bottle there.
Until, when she was about 3 months old, it suddenly didn't work anymore. As if she had forgotten how the nipple worked. After asking the health center, it turned out her 'sucking reflex' had faded, which is normal for all babies, and she fell back on her skills. For our girl, practicing a few times a week was apparently not enough to build up that skill.
Feelings of frustration and helplessness, because if I had known this could happen, I would have given the bottle a few times a day as standard!
And now? I read a lot of contradictory messages online about bottle rejecters. It was also hard to deal with reactions from family and friends (because 'you're a pushover mom, spoiling your child. How hard can it be?'

. I called the health center and a lactation consultant—their advice was 'practice calmly once a day with the partner holding the bottle'. This felt better than 'starving her until she would take it', which according to them also didn't work.
What followed were 5 weeks of daily practice. It was hard to keep it relaxed, because our little girl got tired and frustrated quickly with this bottle that was being 'forced' on her and which she didn't seem to know how to get anything out of. The little moments she could practice (when she was rested, playful and relaxed) were small. At the same time, my tension increased, because soon I had to go back to work and would be away all day. I decided to call my work and involve them in my struggle with this, so I could calm myself down a bit. My work was willing to give me the space to go home halfway through the day to feed my daughter—thankfully one less worry.
We stayed consistent, persisting daily with the same bottle (Dr. Brown, though we had switched to a different nipple stage by then (stage 2)—with a slightly larger hole, suitable for her age). Sometimes we spent an hour, during which our daughter, tired and frustrated, finally had 20 cc inside (this was already a big victory for us). It went up and down; if she drank 10 cc yesterday, it went completely nowhere today. Besides practicing with bottle feeding, we also started at 3.5 months practicing with rice flour porridge with a spoon (with the idea that she would at least get something in if she didn't take the bottle).
She is now 4 months old and over the past 3 days it's getting better and better! As if she finally figured out how it works again! We notice that it no longer matters who gives the bottle or when during the day it happens.
For all parents who are also struggling with this; give your child the time to practice with this daily and as parents, keep going despite the frustration. Give your child the time to master this 'new' skill!