I hope no one would tell you you are a bad mother! Not feeding your child, that would make you a bad mother. Deciding that you need to feed your baby in a certain way does not make you a bad mother
DD never managed to breastfeed so I expressed for a very long time. She exclusively had expressed breastmilk until 5 months when I decided to give one bottle of formula a day to reduce the time I spent expressing. We gradually reduced the breastmilk feeds until nearly 8 months when as my Christmas present to myself I stopped expressing. She still gets one bottle of expressed breast milk a day from the supply in the freezer.
So to answer your questions:
1) Why did you decided to bottle feed?
I had severe pelvic problems and the midwife told me I had to sit up straight to feed and should go to the hospital clinic for breastfeeding advice. The pain of sitting up straight was too much and it was 5 weeks before I was physically able to get to the hospital. My midwife was speaking rubbish and next time I would call to get a supportive breastfeeding advice person. When we went at 5 weeks they got us sorted no problem but by that point she was used to the bottle and to be honest, I didn't like breastfeeding. I was used to expressing at that point so I continued.
I decided to stop expressing a month after I returned to work as I hated expressing and I decided I had given it my best shot and done better than most (I don't say this to make you feel bad, just to explain my thinking

) and I just decided to put myself first. I know that's a horrible thing for a mummy to do but it's what I did.
2) If you started with breastfeeding, how did you stop ?
Before 5 months I was expressing all 6 of her feeds - over a litre. At 5 months I dropped a feed and expressed 5 feeds. At 6 months I dropped to expressing 4 feeds. At 7 months I dropped to 2 feeds and 3 weeks later decided to stop completely.
3) How did you reduce your milk supply so you weren't too engorged?
By stopping gradually. Also I became pregnant and my supply halved, my breasts were sore and it was horrible. I had a m/c and put in the time to build my supply a little but not back to what it was (hence dropping 2 feeds at 7 months)
4) How much do your little ones take each feed and how often ?
Since around 5 weeks she took 5x 6oz bottles a day and one 8/9oz. But that was breastmilk, she has been taking less on formula. We still make up the same amount and throw away the rest.
5) How did others treat your decision? Family, friends, heath visitors ??
Generally that I was crazy to express so much. But as my baby is in cloth nappies, we use proper carriers, my favourites being wraps and we generally revolve ourselves around nurturing the baby rather than making her fit our lives their opinion that it was crazy was part of a package deal of things I did that they didn't understand. People saw moving to formula as the sane option - my parents had been trying to convince me for months, especially after I decided to decline their offer of taking as on holiday abroad as with all the expressing, storing milk etc. it just seemed like too much stress.
6) How did you feel about it and how do you stop feeling so guilty?
I'm OK. I feel jealous every time I see someone breastfeed but I can deal with that. Guilt is a very person thing - if DD gets ill I will feel guilty for not breastfeeding her but while she is healthy it's fine. She got her first cold not that long ago and I do think the reason is because she wasn't getting breastmilk (I thought OH was still giving her a bottle a day from the freezer and he wasn't at the time, I don't know why). I am sure that breastmilk helped her to stay healthy. If you are feeling guilty every day then you have to find your own way to deal with it. For me that was expressing all her milk. For you that may just be taking a moment to realise how much more happy you are when not breastfeeding.
Is he still sleeping through the night? Lucky you if he is - DD has only ever managed that twice

Though she does go to bed by 7.30 and get up between 8.30 and 10.30, it's just she wants fed a few times in between. So if you think that is the effect of formula that's aiding sleep then maybe thinking of that will make you feel better.
We all know that breast milk is best for baby - that's why you feel guilty. But there does come a point when you have to put yourself before baby - if your lack of iron and breastfeeding causes you so much distress that you are not able to be there for him as his mummy then you need to balance that and decide what is best for your family. Formula isn't poison, it's just milk from another species that has been fortified with things. My brother only ever had formula and he is a sporty fit doctor! When we were babies the idea was that you breastfed for 6 weeks at the most then went on to formula.
Now, what you haven't asked are the advantages! You can feed on a bus, you can get OH to do a feed while you sleep, you can go out and do something, confidently leaving LO with someone else without worrying that you have to be back in a few hours. You don't have to stress about whether they are getting enough because you know how much they have.
As a heads up, Heinz Nurture stinks and makes poos from the Bog of Eternal Stench!