I started the weaning process on Friday and my girl is 100% bottle fed formula as of Sunday morning. This was the baby who refused every single type of bottle & sippy cup on the market - I have been trying to wean her since she was 2 months old.
The biggest piece of advice I can offer which helped us is to pick one bottle which has a soft teat (nipple), quite wide, and stick with it. Don't offer any other types of bottles. I made the mistake of trying a different bottle each day, I honestly think it confused Kirsty. I didn't give her the opportunity to get used to one.
I started getting consistent with offering a bottle of EBM once per day about 1-2weeks ago. Pick a time where your baby isn't really really hungry, and isn't tired. Our most successful time of day was just after her afternoon nap. Kirsty really didn't drink much at all from the bottle to begin with, she more mouthed/chewed the teat, but that's ok, let your baby explore it on her terms. It's completely new to her. Even if for a few days/week she just chews it but drinks nothing. It may help to give it to her empty so she can play with it, babies explore through play. Just be careful she doesn't suck too much as she will be taking in air.
Be consistent daily offering her it with BM in. If my girl can do it, any baby can do it! I honestly thought I would be BFing her until she was 18yrs old!!! Once Kirsty took 2-3oz each day, I did this for a week, then on Friday last week I decided I would pump 2oz BM to mix with 2oz formula, and offer her a bottle for every feed - if she took some then FAB, if she didn't take any but she tried, then FAB! And I would have offered her BF after, but I didn't need to.
Saturday we did 1oz BM with 3+ oz formula, and Sunday was 100% formula. Be positive with her, it's very easy to get frustrated and "why wont you just take the darn thing", it's really difficult to keep calm. But more positivity from you and upbeat "yummy!" will work. If Kirsty wasn't happy about taking the bottle I would pretend to suckle it (obviously didn't put it in my mouth), and it helped her realise what to do with it.
Does your LO sit in a highchair? To begin with I sat Kirsty in her highchair to feed her the bottle - so she wasn't confused by being close to the breast. Or being held in the nursing position.
Sorry this is so long - I just feel your frustration! It's taken me such a long time to get to this point, but once the ball started rolling, so to speak, it happened overnight. I couldn't be more proud of my bubby!! It will happen for you, just hang in, keep positive, calm & consistent!