Feeling down and losing libido are symptoms of low oestrogen levels, but it is strange that you should be finding this now, after 15+ years of no periods.
An absence of periods implies low oestrogen - you need a certain amount of oestrogen to trigger ovulation and thereby a period. So we would suspect that you had had low oestrogen for ages and it therefore doesn't make so much sense that you would only be experiencing symptoms now.
Have you had a lot of stress recently? That can have a knock-on effect, sometimes a while after the stress has passed (the body waits until everything's safe and then has a wee wobble).
Another area to investigate would be the thyroid - your doctor can run a thyroid test to see if its action has dropped a little, which again has knock-on effects all over the body. It is particularly involved in energy production, for example.
In the meantime, give your lifestyle an overhaul. Do you get enough rest and relaxation? Are you happy with what you are doing - do you have outlets for your creativity? Is your diet full of fresh fruit and vegtables, with very little that is refined or highly processed?
Do you have plenty of plain water every day and barely any caffeine? Are you running around looking after everyone else so much that you don't have time for yourself?
If there are any areas where you think your body could do with more support, give them some attention - it's a very good time to focus on your own health to avoid feeling you're losing control.
Things that may help you include Kelp and Salus Haus Epresat, both of which give you nutrients but also an energy stimulus.
You could also consider using Hypericum, so long as you are not on any other medication. It is very supportive during times of low mood, and doesn't have the side effects associated with medication for low mood.