YANBU, I wasnt keen at first but the only thing that kept me going was not having to get up in the night to do bottles
[lazy] . Have now been doing it for 8 months and do enjoy it all now, especially as he is so efficient at he only feeds for about 5 miniutes.
While I am all for bf, I am more for "do what is best for you both". It is absolutely 100% fine to feel the way you do, and while BF is the best source of food, formula is the next best thing and does the job perfectly well.
FWIW we started mix feeding fairly early (albiet a bit lazily, some days he had one bottle, some 2, some none at all) so I did risk cocking up my supply, but I used to pump aswell from time to time which helped to keep it up. Maybe if you want to introduce mixed feeding now you could pump aswell to help with the supply? It does seem a bit daft to BF, FF and pump BUT it gave my DP a chance to feed baby while I skulked off upstairs for a lie down and I just pumped when I started feeling my boobs getting a bit full.
Also boobs are amazing things, if you do introduce mix feeding and you feel your supply dipping when you do BF (you'll know if this happens, baby might piss about when feeding, get frustrated, wont seem satisfied, strum your boobs etc etc) then you can just put him to the breast more often to get supply going again. I was a bit ill and my supply dipped dramatically so I took off upstairs with baby and a book, top off, boobs out and just let him suckle whenever he wanted. Took about a day of this to get it back.
WRT not knowing if she is getting enough, I thought this aswell so did a little experiment [nerd]. Look at a bottle, the size of the hole the milk comes out of is teeny, put a bit of water in the teat and push it through and it comes out in a teeny tiny little stream. Now get your nipple and express a bit out. It doesnt come out of just one hole, but about 6. So by my reckoning if it took DS say 20 mins to take 4 ozs, and he only bfed for 10 mins he was getting far more out of 6 holes in 10 mins than he was in 20 mins out of one hole!