I wish I could give you a great big hug!!! Bless your heart!
First of all, no matter what you decide, your son will grow up and be a wonderful, beautiful person. Breastmilk doesn't "guarantee" perfect children. LOL! Don't let anyone tell you what you do is wrong, because it isn't...it's right for you and your family.
If you have been dedicated to the idea (the idea is different than the practice) of breastfeeding, then it is a very hard thing to give up. If you haven't done so, see a lactation consultant so that she can weigh your baby with her special scale before and after he eats. It could be that you are simply not producing enough milk quickly enough. This happened to me. My firstborn didn't gain weight like he should have (he was eating every 2 hours or so) and we found that I just didn't make enough milk. As a first time mom I had NO idea what it felt like when my breasts were empty vs. "half full" if you know what I mean. (I knew "full" of course.) So at 2 weeks old, we had to change my Perfect Mother Breastfeeding Plan.
Our solution, which broke my heart because I was sure I was putting poison into my baby, was for him to nurse 15 min on each side like they say, and then give him a 2 oz bottle. Daddy actually loved the bottle part because he could feed the baby. I also took an herb called fenugreek that stimulates the production of breastmilk. (I think there's a prescription, Reglan, which also does that...but it wasn't FDA-approved when I did it) The fenugreek did help. By the time my son was 6 weeks old and I was returning to work, my milk supply was better. (Irony!!!)
However, I could not, for the life of me, pump enough milk to supply him while he was at the sitters. So I would pump what I could and then supplement with formula. We did that for 4 months till summer came and I was at home with him. Everyone survived...I think I was the worse for wear because I felt so guily. Even then, though, I could see the blessing in it all because he took a bottle so nicely. (He did NOT take a bottle nicely again when I returned to work in the fall, after exclusively BF during the summer, LOL)
The other thing I did, which I might get bashed for, is that I took that baby with me to bed. I am a fairly light sleeper, but I needed to sleep! I'd sleep turned or half-turned on my side so that he could nurse, and nurse he did...ALL NIGHT off and on. I'd switch sides when he got restless and that side was empty...and go right back to sleep. I protected him in the crook of my arm (my shoulder was the "corner" not the elbow) and Life Was Good. The doctors and lactation consultants knew and condoned this...they said it is a phenomenon in working moms called "reverse nursing"... It worked for us and I have no guilt at all. :)
He contined to nurse on demand until he was 14 months, when I got pregnant again,) although the reverse nursing stopped when it was summer and I was home with him, and he did sleep in his own bed from 5 months on. :)
I was in my early 30's when my son was born...so like you he was a long awaited gift. You won't hurt him, or you, with formula. It sounds to me as though you are not seeking to stop nursing, just supplement...and I will tell you, it's the perfect solutuion (as long as his tummy tolerates the formula...what we did do, I am vaguely remembering, is that I pumped - mostly to increase production - and we did 1/2 breastmilk and 1/2 formula - for the first few bottles, then decreased the breastmilk and increased the formula till he was at 100% formula.)
(FWIW, I nursed my second child for 2 years and never ever had a production issue...I had to throw away tons of frozen milk after she stopped nursing!)
And don't let ANYONE tell you that you are wrong!!! Breastfeeding is a huge deal and it is HARD HARD HARD! (It's also SUPER easy, once you get it going...I loved not having bottles to drag around, etc...) And don't let anyone tell you you're "not committed" or "want some time for yourself" because I will tell you this...you cannot lose YOU in the midst of parenting...that's how you become unhappy. Go out to dinner with your husband, with your girls, go to the mall without the baby, be a Woman too, not just a Mom...because when you go back home, that baby will be fine, and YOU will be better, because you took care of yourself just a little bit. It's not selfish, it's smart!!!
Hugs to you! Good luck with breastfeeding!