A few comments:
You said: "As my recovery from depression has progressed, I have returned many time to my practice, but I continually abandon my efforts--sometimes after a class or two, sometimes after six weeks or a few months. Instinctually, I return to yoga again, yet I am absolutely petrified. In addition to my loss of physical facility with the postures, I have gained a drastic amount of weight (seventy pounds)."
If you have been away from the practice for several years and have gained weight, then of course it is going to be more difficult. That is only a practical, mechanical matter of getting the body back into shape and back into the habit of doing the yoga practices. It will take time, don't expect instant results. Don't worry about doing the "full" practice as others in the class may be able to do. Do what you can do and don't compare what you can do with what others can do. If you are persistent and patient, then your body will adjust over time and you will find yourself being able to do more and more. There is no such thing as "I can't", there is only "I can... even if only a little bit... and as time goes by, given enough effort, I will be able to even better." If it's something you really want to do, then do it, and don't give up... but don't be unrealistic in your immediate expectations, and don't push yourself too hard. A slow but steady advancement is much more beneficial than a rapid, unstable advancement that is often followed by a hard crash or "fallout"...
You said: "I feel betrayed and hurt that my practice was not "enough" to stave off my depression."
There is absolutely no reason to feel "betrayed". Yoga itself is NOT enough to stave off such a depression. When strong karma is ready to unfold... or to put it another way... when karmic seeds are ready to sprout... there is very little that can be done to fully prevent that unfoldment. There is much that can be done to mitigate the effects, but very little can be done to fully prevent it. Yoga is one tool, or aid, that can be used to mitigate the effects of karma. But it is no "super-cure" that will fully prevent depression or other negative karmic tendencies. Other such "tools" are meditation, prayer/worship, service to others, contemplation, proper diet, study of spiritual texts/scripture, avoidance of "bad company" who will only drag you down into negative states of mind, and others that I'm surely forgetting to mention.
Your depression was an unfoldment of karma, possibly from a strong habitual depression from a past life, or due to poor "choices" or circumstances in this life (such as diet, living conditions, the company you choose to keep, low self esteem due to comparison to others or unrealistic expectations of yourself, and many other possibilities). It is something that must be accepted, and worked through. It sounds like you are coming out of "the worst of it", but I'm sure you are still feeling many negative after-effects. The important thing is to not let those effects bring you down. Accept the fact that they are there, but don't give in to them. Don't let them control you. Pick yourself up and push on, no matter how many times you fall. It requires effort and willpower to lift yourself up from muddy places, but if you keep trying, you will make it out for sure.
I have read that "No amount of spiritual effort goes unrewarded", and I have seen it to be true. No matter how miniscule or seemingly insignificant your progress is, and no matter how many times you fall, whatever effort you make is NOT lost. You may not notice it when looking into the immediate past or present. You may not feel that you are making any progress at all. It may take 5, 10, 15 years until you can look back and actually notice any significant progress. But I assure you it's there.
So don't let "failures" and negative feelings bring you down. Those negativities are just dirt and grime that need to be cleaned off. They are not a part of you... don't worry so much about them. If you focus on the negative, "dirty" things, then that will become all that you are able to see. Focus on the positive things that you enjoy, and don't pay much attention to the negatives, no matter how strong they may be.
There is probably a lot more that I could say but my thoughts are starting to wander, so this is all for now. :)
Good luck, and don't take things so seriously... life isn't as serious as people tend to think. :)