Keep going!
Don't give up. You will get there. We are all capable of anything, certainly singing and playing guitar. It's like when Ray Manzarek played keys for The Door he used to play the bass part in his left hard and chord/melody is his right hand. The way he described was like 'splitting' his brain in half. Letting the left-hand go into auto-pilot.
The best way to do this is to get really confident with your guitar part by practising slowly in time. Once you manage to play the guitar with fluidity, building up the tempo, start to do the same with vocals. Really get to know the lyrics and melody, and practise them religiously by themselves. Once each part is instinctive bring them together and practise them slowly.
This may all seem a bit of a long process but if you start of this way then you will get use to singing and playing at the same time that you can 'split' your brain and play any song without having to think about it too much.
Keep going you can do it! Once you get it will be so rewarding.
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