I started with a group some times ago a new campaign. I made a thief that used a bow and was very very stealth based (level 3 thief having +21 on stealth with all effects applied). The intent was using the stealth to get sneak attacks with the bow and using mastersniper to remain stealthy despite shooting around arrows.
I asked the DM if they were fine with a stealth loaded character which they agreed to.
Now our 5th session went by, and my thief so far got knocked out 7 times in these few sessions.
The thing is, my character is never being allowed to use stealth, when in crowded areas I am told "you can't use stealth here, too many people could see you" In empty places when I say "Ok, I am hiding behind the corner of that hallway, waiting for anyone possibly joining this hall, while our group discusses their actions.", I am told "It's an empty hall, someone joining it would see you anyways, you should instead join the groups discussion" Or even in our very first session when our group was walking through the desert I said "Ok, I am trying to find a low profile in the desert, walking close to the group but staying unseen for anyone in the distance." to which the GM responded with "You all are walking through a desert, its inescapable to be seen." I even was more than once having been revoked my stealth state after a successful stealth check just by passing an enemy's line of sight during combat, which, as I understand from the rules isn't even how it is supposed to be, as the previous stealth check determined that exactly that is not gonna happen.
And what adds even more frustration to it. Our enemies always engage us stealthily. There wasn't a single encounter so far, which wasn't approached by us where we didn't have to roll perception against their stealth. In the before mentioned desert situation, I was complaining "How can we even be sneaked upon if I was told, I can't use stealth here cause we would be seen from very far in any direction." to which the the GM responded with "Well rather than your group the enemy knew the area, they sneaked onto your group using the cover of the dunes next to where you were walking to gank on you."
Given all this, I think I don't need to explain why exactly this is very frustrating for me... my character is simply denied the use of the only thing he is good at, while all others at the table can use their characters' features as intended. I already confronted the GM about the situation and asked them if they could opt for letting my character have more of their moments. What they said was, "The first sessions are in very open areas, you were aware of it, so you shouldn't expect it to work, but as soon we are in the jungle where this mission is supposed to get to, you will be able to make more use of it." So the last session we finally arrived in the jungle and it just continued "You can't use stealth here, the group is walking on an open path", "You can't use stealth here, the plants are so thick that one would be able to trace you through the undergrowth anyway" and so on.
So my question here is not about whether the GM or me is right in their way of how to expect stealth to work. But rather:
How can I effectively communicate to my GM - without making them get defensive - that this is just boring, very frustrating, and not fun at all for me?