I've been having nightmares for two weeks now, and now I think that I'vem the reason why this is happening.

- By adding 'now' to 'I've been having nightmares for two weeks', it puts emphasis on the fact that before these two weeks you weren't having nightmares but it's now been two weeks that you've been having them. You'll sound native at least in your sentence construction if you do things like this. You did this perfectly in the next part of your sentence when you said '... and now I think that ...'. You could have written '... and I think that ...' instead but by adding the 'now' as you did, it puts emphasis on the fact that there has been a change in situation, specifically that you now think you have the answer to why you have been having nightmares where as before you didn't have that answer.
- You could change the ending to sound more sophisticated by saying '... and I'm now convinced that I'm the culprit'

I didn't know how stressed I was until this happened.

- Adding 'happened' to the end just makes it a bit more clear that you are specifically referring to the nightmares you have been having for two weeks

In one month, I'mll be starting my last university course, p. People think, as meI had in the past, that the last course is the easiest one, however, it isn't beI don't think this is the cause I have to find a jobanymore because to validate somthe subjects and I should get my TFG finished, I have to get a job.

- I'm is the contraction of I am which is present tense, so to write 'In one month' where you indicate you are about to talk about the future and then have the be followed by the present tense with 'I'm' doesn't fit. Use the future tense either 'I'll ...' or 'I'm going ...'.
- Full stop between '... my last university course,' and ' people think ...' because they are two separate ideas. If you don't want to use a full stop, you could also use an 'and' for example 'In one month, I'll be starting my last university course, and, people think as I had in the past that ...'
- as me in the past -> as I had in the past
- Technically no problem with 'easiest one' but really what you are saying is 'easiest course' which in English to avoid repetition you can just drop 'course' if it's clear that's what you are referring to. Just having easiest instead of easiest one makes it sound more fluid
- To be honest 'it isn't because I have to find a job to validate some subjects and I should get my TFG finished.' doesn't really make sense. I wrote what I think you meant to say. The reason I have written 'I don't think this is the case anymore' is because you're showing you have changed your idea but also showing incertitude because you haven't completed the subject yet so you can't be 100% sure.

Finding a job is for me is a new experience, I'm really afraid of being rejected of any company or not being able to find iby everyone. To be honest, it's really stressful, to be honest.

- Technically fine but not super natural to write 'is for me' in English, order should be 'for me is'
- In the context of context of being scared, (at least to me), the word afraid more so replaces 'really scared'. I think it's more natural to just keep afraid. If you want to emphasis your more than afraid then you would use a word like terrified but like that's very strong and if you said I'm terrified of being rejected I would expect you to be shaking in the interview if you were truly terrified.
- People will get from context that by 'everyone', you are referring to all the companies you will be applying to. This just makes it shorter, more concise and digestible
- No need to write ' or not being able to find it' because the start of the sentence you make it clear you are talking about finding a job
- Not very common to end a sentence with 'To be honest'. Putting it at the start is much more natural

Finally, doing a TFG is really hard as far as some of my older friends than me have alreadyhave told me.

- bit clunky writing 'some older friends' in English, you would normally always specify whose friends which in this case you mean yours
- You are using the past tense 'have told me' so there is no need to have 'already'

I'm fucked up haha.

Being stressed about courses, finding a job etc are all normal things to be stressed about. You're not fucked up because of it. If you were then everyone on the planet would be fucked up

TWriting this is my way to feel more relax by expressing my feelings.

- Added 'Writing' at the start to be explicit. Without it the readers are wondering what exactly you mean by 'this' because you haven't referred to it anywhere before.

I've been having nightmares for two weeks and now I think that I've the reasonI know the why this is [may be] happening.

may be = more natural

I didn't know how stressed I was until this happened.

In one month, I'm starting my last university course, p. People think, as me in the pastI used to, that the last course is the easiest one h. However, it isn'tthis isn’t true because I have to find a job to validate some subjects and I should get my TFG finished.

Finding a job is for me a new experience, for me, and I'm really afraid of being rejected of anyby a company or not being able to find itone, it's really stressful, to be honest.

Finally, doing a TFG is really hard, as far as some older friends than me have alreadI have heard from some of my told me.er friends.

This is my way to feel more relaxed by expressing my feelings.