I could have written your post! I just recently gave up my soon to be 4 year old Golden retriever 2 weeks ago. It was one of the most difficult decisions I've ever had to make. [:'(] In fact, I wanted to post about it, but was still was too emotional and was afraid people would think I was irresponsible for giving her up. A little background...
We bought Niya as a puppy. I flew clear across the US to Niagara Falls, Canada to buy her. She was a light colored show quality English Golden Retreiver and I loved her to pieces. She was VERY, VERY high energy. We obedianced trained her. We crate trained her. We walked her. I even hiked with her. She was good on those days that she got her exercise, but still very hyper and out of control (would often run by and knock my 5 yo ds down in her excitment) when she did not. When I became pregnant last January, the hiking stopped and it was difficult to walk her as I suffered from a lot of ligament pain. Around April I began thinking about whether or not it would be best to find another home for her, but I really wasn't ready. I too, feel that when you buy an animal, it's a committment for life.
Niya was usually confined to the kitchen and only part of the family room. She hardly got any attention. When my baby was born in September, I knew that because I loved Niya so much, I had to let her go. I put the word out that I would like to place her with the perfect situation. I wouldn't just give her to anyone, it had to be a better life for her. In October, one of my friends was chatting with a friend of hers and come to find out they were looking to rescue a Golden Retriever. Their Golden had died nearly a year ago and they were at the point where they felt ready for another dog in their lives.
They have 2 older children (13 and 16) and the husband works from home. They came over the next week to meet her and we talked at length to see if this would be a good fit. It really couldn't have been more perfect.
Niya has been with her new family for 2 weeks now and she is doing fantastic. They brush her daily, walk her at least twice a day and she is really the center of that family's world. The mom has called me twice to teel me how she's doing and we have an agreement that when they go on vacation, I will keep Niya for them. I am happy to still be able to be a small part of her life. it was sort of an open adoption, if you will. 
It was not an easy decision. But it was what was best for her. I couldn't be selfish and think about how it would make me feel, I really had to put my dog first and do the right thing for us at this time. My kids too were a bit sad, but they really did understand.
Good luck in you decision and please IM me if I can offer you any support or advice. 