I understand why it is difficult to approach her. Good friendships can be hard to find and she has been a good friend to you so you don’t want to risk it. But, on the other hand, most things that matter require some degree of risk.
You say she has been a good friend to you. Have you been a good friend to her as well? If not, that is the place to start. Make her life brighter and easier in any way you can. Help her believe in herself just as she has helped you. Be a positive presence in her life. My guess is that things should move naturally from there.
If, however, you already have established a balanced relationship, it may be that each of you is waiting for the other to make the next move. You could tentatively and respectfully ask her if she would be open to seeing each other socially. Keep it light. Take it slow. Don’t start talking about “settling down” until you get a clear indication that she feels the same way you do.
I hope your patience is rewarded.
I wish you well.