GRADUALLY!!
Unfortnately due to lots of rubbish things I could only breastfeed for 6 weeks and had never even considered formula feeding when pregnant which was naive. After 6 weeks of breastfeeding, I did a further two weeks of combi feeding via a breast pump (struggled so much with the actual boob) and Aptamil formula. By 9 weeks we were on formula only.
I thought that the minute I gave my son formula, all hell would break lose in the tummy/nappy department, he would be ill, upset, etc. I was DREADING it. I got really anxious and upset and felt I was failing him, and you know what, nothing changed. He remained his usual self, nappies didn't change instantly (nor did they get obscenely horrid like I was expecting) it really wasn't that bad for all the worry.
Just be so gradual to be easy on your baby AND yourself.
I had about 3 days of waking up in pain with my breasts but it does go eventually. I pumped milk if things were unbearable. One day we had a daytrip out, I had no oportunity to pump, and that kind of became a cut off day. I tried really hard from then on to just 'get on with it' and not pump anymore, and really quite soon my boobs were back to normal. Wear a decent tight bra. If you think you are getting lumps and bumps express to try and avoid blockages or mastitis. You've done great by the way, well done xx
ETA: also regarding bottles maybe give those closer to nature ones a try, although all babies are different and you might need to try a few teat types and shapes. Mine only coped with the standard neck basic teat shapes, contradictory to what 'should' have happened he couldn't get on with the more naturally shaped ones.