Please do not be so hard on yourself about the real problems you had with breastfeeding. Some people DO find it this hard, and you absolutely should not feel guilty for having had to use expressed milk and then formula.
Your story is so similar to mine. Despite taking up all the help offered from numerous midwives and the health visitors, and all of them saying I was doing it totally right, checking it was the correct latch and position and so on, feeding was still excruciatingly painful and I dreaded the pain, throughout each feed, which made me cry.
I really took it to heart and was incredibly upset as I had so hoped to breastfeed. I'd very much taken on board the "breast is best" message which I'd seen promoted at every turn, and felt there must be something really quite wrong with formula and that I must BF at all costs. I felt like a miserable failure!
To cut a long story short I did have to move to expressing (alongside several more weeks of unsuccessfully trying to BF without awful pain) and then formula just as you did. A couple of people finally at this point were really kind and said that I had clearly done my best. This was so reassuring as I knew that was true, yet was feeling awful at being unable to BF.
When I eventually stopped the expressing and moved to formula I suddenly had an extra few hours in the day. I'd been expressing/feeding/sterilising for over an hour out of every 3 hours day and night. It would take 40 mins or so to get enough milk for a feed, 20 mins to give the feed and change baby, another 15 to wash and sterilise the pump and put it back together for next time. On removing these tasks, obviously it was fantastic to suddenly be able to spend much more time just playing with the DC, giving the baby attention, rather than being wired up to a pump so much of the time. It was an amazing and positive difference, and this is what turned out to be "best" for us.
I know you want to treat your children equally, and that's of course a very good thing. However "equal" in this case for you could be choosing the feeding method that you think is best for your baby, you and your family, in all respects, at this particular time. This could be trying again to BF and seeing if it's better this time; or aiming for a certain (do-able) amount of expressing when baby arrives, or FF from the start.
If anyone else tries to criticise, just ignore them. You have every right to make your decision about this.
Best of luck whatever you decide.