OK. First on the depression (I'm a medicated depressive) - I believe that each of us have a responsibility to take care of our mental health and use the help available to be the best we can for our relationship. Anything less should be unacceptable. Anyway, the lethargic/lazy/apathetic thing is 100% symptomatic of the depression. I don't know much about weed, but I think there's a fair chance it's making matters worse. It doesn't matter how much you try and motivate her to do something, the depression will make any task feel overwhelming and she won't be able to do it. I would advise you to stop trying to motivate her into exercise and put your efforts into addressing the depression.
Speaking of motivation, you're expecting too much if you want partner who motivates and inspires you. Just as she is the one who needs to be able to motivate and inspire herself, you too, must be able to rely on yourself for motivation. That being said, it's important to avoid a partner who drags us down. Simply having a partner who leaves us in a good emotional state so that we can inspire ourselves to do our best work is all we can really expect from another.
Anyway, I think the way forward must be with some tough love. You say that you love each other and want the marriage to work. Let me ask you this: if she doesn't get her mental health under control, are you willing to stay regardless? If not, this is the point where you need to start. She needs a massive kick up the rear end along the lines of "this marriage isn't working for me at present. If I am to continue in this marriage, I need you to start to take responsibility for your mental health - including counselling and proper medication" However, if you're not prepared to walk away if she stays the same, then you can't throw this threat out there.
You mention that you are a musician and do the gym 4 days per week. Does this mean that you're absent from the home during these times? (practicing for hours on end in another room also counts as being absent). Just as she's got to make sure that she gets her mental health under control, you need to make sure that you're *present*. It's hard to make a marriage work if a partner is gone too much. I'm only guessing about this - perhaps you're fitting all this in and still managing to be around. Just throwing it out in case you're not.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.