Welcome to Beyond Blue and well done for reaching out.
I too had to rehome a dog I had, many years ago, and I felt that same guilt that you describe. I felt I was letting her down terribly. It had never been my intention to have a dog for a short time - we only had her for 18 months. But same reasons applied for us; we were renters, and moving to a place that wouldn't allow pets. She was a German Shepherd, and her name was Sam. I still have photos of her, thank God, and I have never forgotten her. She will be forever in my heart.
Just allow yourself to grieve the loss. It's okay to be sad when things don't go as we had originally planned. And remember, you are doing what is right for them; giving them the space and environment that they need, instead of selfishly keeping them 'locked up' in a small space where they would become bored and destructive. What you are doing, therefore, is actually the right thing for all of you.
Having said all that, I know it's hard and sad. but you must remember; you are doing the right thing.
Be gentle with yourself, and take lots of photos of them! Fur babies will be forever in your heart, even though they will be living elsewhere.
Take care. I'll be thinking of you. (((hugs)))