It does take time but eventually you are soloing a greater rift level 50 or higher and it's been a nice and rewarding progression. It's not just about finding the best gear, well yes it is I guess, but you also have to play to your strengths and find the right strategy.
Make sure you are taking advantage of your active skills, runes and passive abilities. Mix and match with different things until you feel your character is good enough to handle a big mob of elites, much less the greater rift guardian, while soloing a high level greater rift. If you can do that then going to public games on Torment VI will be a breeze and feel like "normal" mode when you first started playing. Others will be asking you for help.
Be patient and have fun. That's how I became great at Diablo III. If you feel discouraged or bored then it's hard to make progress.