Ok, we went through this forever with Abby...one, get her checked. It turned out Abby had giardia when we resecued her...meds helped a LOT!!!! But, she was still very difficult to train. While she quickly stopped pooping in her crate after getting over the giardia, peeing was an issue for a long time. probably at least once two weeks, we still deal with it and she's almost 18 months!!! A few things though...first, let's address the issue of spearation anxiety. It is very possible...at that age, they get nervous being alone and that is the result!!!! I took Abby with me EVERYWHERE! When I showered, I closed her in the bathroom with me. Running into the kitchen quickly? Take her with me, etc. Now, if I know she has recently done her business, I can let her roam free for a while...but I still have her with me in the same room about 80% of the time. And immediately before I allow her to roam free, she MUST pee. I still keep her with me while I shower, unless, again, she JUST went. If I allow this, I do leave the door wide open so she can stay with me if she wants...maybe just knowing she CAN see me if she is willing to stop playing for a few minutes is enough.
With respect to crating though, obviously, you can't keep her with you so you need to address those issues separately. The night before I go to work, I fill a kong with peanut butter and freeze it overnight. Immediately before I head off to work, I tell her to go in her house, and she gets the kong. It works wonders for us...rather than her getting all worked up about us leaving, she spends a good 30 minutes or so trying to get all the peanut butter out. By the time she's done, she's comfortable with us not being in the room with her and while she may whine for a few minutes, she's now tired and off to sleep she goes. Spending some time with her whiel she is in her crate is useful too...make a few nights of sitting right outside it (door open), with my legs inside the crate and her in my lap was very useful for us...she didn't see the crate as a "bad" thing when I was hanging out with her in it....
Stopping her from peeing in her crate altogether is still difficult even once well adjusted to being left alone and crated. Once they are used to going in it, it is a hard habit to break, so you basically need to make them get disgusted with it. You want to keep the crate area as small as you can (use a divider)...I think the standard is, big enough for them to turn around and lay down. Thus, if they go, they HAVE to deal with it the remainder of the day (and of course, you then have to bath them!!! gets annoying but breaking this habit is most important).
For a long time I stopped putting her crate mat and towels in there...so that the fluid would not get absorbed...she stopped going in her crate. Then I started to add the crate mat back in...and found that she started peeing again. I just washed it every night and right back in it went. She eventually got the point and now, rather than peeing everyday, we are probably down to about once every two weeks.
Good luck, it takes a LOT of patience...but 6 months is rather young for a boxer to be capable of handling everything. SO wait it out for a while and just keep being consisitent and patient.