I'm trying to practice playing songs on guitar while singing. Thing is though I'm not exactly trying to be rhythm guitar, I'm planning on playing my own solos and everything, but what I noticed is that it's somewhat easier to play and sing to my own works compared to cover songs. Currently I'm trying to learn to play Fuel by Metallica, I know most of the guitar parts, combining the rhythm and lead guitar parts most suitable for one guitar, and it's much more difficult for me to play and sing it. I have looked at the thread about this matter, but I'm wondering if there's any other practice techniques to help me in this matter besides just learning to sing the song correctly and try my best to play it while focusing on my voice.
Play it slowly, and make sure you can play the parts without thinking about it.
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singing and playing requires the same skill as playing the drums in my opinion, so if you have drum experience then that would help you appreciate the mental effort that you have to give.

you may already know this, but I thought it could be interesting to highlight this relationship
try just saying something on a certain beat of each bar. Name different fruits, vegies, guitars, drinks. And everynow and then choose a different beat o fthe bar to say this. Then choose maybe a few. The hardest beat (for me anway) is the last beat of a bar, holding to the first or second.

But this will improve the more you practise, and in no time you will be able to sing and play at the same time. Also, a lot of beatles songs are very easy to sing and play at the same time, try blackbird.
My dad used to play drums and he plays guitar a little (basic chords and stuff) but he can sing and play just fine so the drum thing is probably true. now me, I can't sing and play and have never played drums but I'm slowly getting better, learning to sing and play whiskey in the jar (The celtic folk version not the Thin Lizzy version)
As someone above said, you have to be able to either play or sing without thinking about it.

The reason you can walk and talk at the same time is because walking is so easy for you that you can put all your thinking power into the conversation. The same is also true even moreso for breathing, we breathe all the time yet we can carry out many other things at the same time because breathing is probably the easiest and most natural thing we do.

So to be able to sing and play, you have to be able to do one with minimal thought which, for guitarists, is usually the playing part. To do this you have to practise the part over and over till you barely have to think about it. It also helps if you don't have to be constantly searching for the words or the tune.

However, the harder the guitar parts get the longer it takes to be able to play them fluently enough which is why most people find it easier to play rythm and sing than play lead and sing.

But singing and playing lead is very much possible, you just have to make sure you can do both parts really well separately before you try and put them together. I wouldn't suggest doing any extra exercises to try and help you because the thing that will help you the most is if you can invest as much of your time as possible in learning the part (which will make you a better guitarist so new parts get learnt quicker).