Hey Mag! It is natural for someone to feel discouraged after countless failed attempts, but with every failed attempt you get better and ever closer to that first LD. If you give up you probably won't have any LD's, but keep at it and you'll have one soon enough. That's the hardest part and after that it's all downhill so if you can make it there I promise you it will be worth it to keep trying.

As for the actual things discouraging you, it's good to settle in and try one method for a while so your body gets used to it and forms a routine, that is a key for LDing.

Sometimes it's unavoidable to spend countless hours in front of a screen, especially if it's part of your job, but as long as you have a period of an hour or so in natural lighting before you get ready for sleep that shouldn't affect anything.

If you can't fall back asleep after your WBTB try setting an alarm for an earlier REM period or if you are getting up and moving or eating, maybe just lay in bed and relax until you're ready to do your MILD so your body stays in "sleep mode"
When you're doing your MILD and repeating your mantra as you drift closer to sleep repeat your mantra less frequently only repeating it if a stray thought comes to mind. Let your self fall asleep, instead of fully concentrating on your mantra, keep it as a governing thought in the back of your mind as you fall asleep. The problem could also be with your mantra, make sure it is something you find truth in maybe something more general instead of something direct like "i am dreaming"

Don't concern yourself with finding a dream sign, that can take plenty of time, just keep journaling and re-read your entries while looking for any consistencies.

Good luck and don't quit, anyone can do it and you're almost there! Peace