Oh, Guimauve, you sound so upset about this, and you are being really hard on yourself. I know excessive guilt is part of a mother's remit, but please listen to everyone on here.
You have BF your baby for 5 months, which in MN terms seems relatively little but believe me, is 5 months longer than most women around where I live. You have given H your milk, which has provided him with a clean bill of health, and which will have given him invaluable immunity and antibodies. You have NOT starved him, on the contrary.
The palaver of formula will get easier. And as someone said, he'll soon be onto solids: if, as it seems, your baby just isn't into milk, he'll grow like mad once he gets onto bananas, avocados, porridge, etc. You've done all you could and more, and your instincts are spot on.
And you are NOT being over-indulgent; everyone is talking sense to you, but most if not all of us would feel pretty much the way you do in your situation (cf above re excess guilt...!).
Have a huuuuuuuuge glass of wine/whisky/ G&T, or whatever your particular poison is.