I played a DH to 70 and got on adventure mode. I ran a NR with a guy and joined his party. He logged and another guy appeared in the rest area and the screen popped up to see if I wanted to join a Greater Rift so I clicked OK. They guy ran the rift full speed and I just tried to keep up while every once in a while shooting at something and thinking I was doing a good job. Turns out it was a level 150 GR and I went from level 1 to 250 and got some nice loot. Same thing happened the next night and before I knew it I was level 600 and my paragon screen was almost full.
Now when I solo I can only beat level 25 GR and I never get any upgrades compared to the loot that I got from the level 150 GR. My character isn't strong enough to solo rifts above 25 and I'm not progressing. What am I doing wrong? Getting stale to run these NRs and GRs and get nothing but new gems that only go in my jewelry slots.
Depending on how many runs you went through and what loot dropped for you, you can be stuck anywhere between GR25 and GR45.
What I have noticed is that "fresh" players aka noobs tend to make the wrong choices when rerolling stats from the mystic. Example: if they are running a typical build revolving around Crit hit chance (CHC) and Crit hit dmg (CHD) but dont have either of those stats on gear, they roll off a secondary stat for a 2% chance to freeze / stun.
It also depends on what you are gambling for at Kadala. I always recommend to new players to gamble for armor pieces with kadala as they only cost 25 shards. never gamble for weapons or jewelery until you are looking for mynute upgrades or plans on using "hope of cain" from kanai's cube.
Also make sure you find the kanai cube in A3 and do bounties. With kanai's cube and some correctly roll rare items (yellow gear), you can easily break passed GR25.
I'm not going to give more advice, as there is to much content.
Hit up Desolacer on youtube. His channel is rounding out really nice now a days. He is currently making different builds that aren't META , meant for material farming, or just to show case all the other builds out there.