Well, you have several options if the door itself is sagging.
You can remove it and just hang a curtain across the opening. I removed our 3 section sliding glass doors just a few weeks ago because we got tired of them slipping and falling. I used a bathtub tension bar and hung a shower curtain. Much easier and less costly way to go.
You can also add a wire brace, from upper right corner to lower left corner. You'll need a cable wire, a turn buckle and some way to attach the wire to both corners of the door.
You can square up the frame, then drill small holes at each corner and put in flat L-braces at each corner. Be careful to not drill through the glass or try to insert a screw through the glass. So, drill carefully. When done, a spot of spray paint on each one and no one will ever notice they are there.
Or....
You can take the door frame to a shop and have them reinforce the frame. They will probably strengthen each corner in a similar way, with L-braces, or build a whole new frame for you.
If it were mine, I try the flat L-braces on each corner.
Good luck.