The Effect Of Being Left [or: 'Jilted'] By One's Boyfriend [Question: are we only talking about boyfriends or about girlfriends as well? If the answer is the second, girlfriends will equally need to be mentioned]

Being left by one's boyfriend is a big problem in modern society. There is no doubt that being left by one's [OR: 'a'] boyfriend plays a critical role in people's daily lives. As far as this subject is concerned, it can be easily said that there are some effects [OR: 'consequences'] of being left by one's boyfriend.
The first effect is poorer performance at school. The people being left by one's boyfriend when they could be sad [This sentence lacks a main verb. Perhaps: 'Being jilted can make people sad']. A good example might be given in is provided by researchers done by Scientific Magazines. According to it them %35% of European Union youngsters can be [perhaps: 'have suffered'] depression. As a result, the people might to fail in their daily lives. Apart from this, this situation can affect them in their future life.
The second effect is on new relationships. The people who being left by one's boyfriend can be alone because her boyfriend i.e her someone's right hand give up her [Failure of communication: I don't understand the idea. Perhaps: 'Jilted people can feel lonely because the other person has...']. For example, I broke up with a boyfriend when I started to find develop a new relationship. Thus, I can get over this. [I understand: 'Thus, I was able to get over the break-up.']
The final effect is a feeling worthless. The pPeople is are very valuable but they fallin sadness when sad they can feeling worthless. For instance, my cousin broke up with her boyfriend while she was very sad because she think of her ex-boyfriend was sadness by yourself [I don't understand the 'because' clause, I'm afraid].
Taking these points into consideration, there are three effects of breing left by one's boyfriend upon people in terms of poorer performance, new relationships and a feeling worthless. I obviously claim that the people should not worry about everything because the life is very short.

Hope my suggestions are helpful! An ambitious piece of work. Perhaps you should re-read your work before publishing it, so you can rid it of unnecessary minor errors. All the best!

The Effect Of Being Left By One's Boyfriend Dumped by Your Partner

This is a little colloquial, and perhaps Nick's suggestion of <em>jilted</em> is slightly better. The phrase "being left by one's boyfriend" sounds awkward and implies that the other person in a relationship is always a man. ;)

Being left by one's boyfriend Having your partner leave you is a big problem in modern society. There is no doubt that such an experience being left by boyfriend plays a critical role in has a serious effect on a person's day-to-day life. As far as this subject is concerned, it can be easily said that there are certain effects of being left by one's boyfriend deserted by someone you love.

If this were for an IELTS exam or another English certificate, you would certainly lose points for repeating the same (awkward) phrase. In a general article or blog post, such repetition is simply dull and makes your text unpleasant to read. Try to find different ways of expressing the same idea.

The first effect is a poorer performance at school. The people being left by one's boyfriend when they could be sad People who have lost their lover are usually quite depressed. A good example might be found in researchers conducted by Scientific Magazines. According to the study, up to 35% of European Union youngsters in the EU can suffer depression. As a result, these people might continue to fail in their daily lives. Apart from this, This situation can also affect in their future lives.

I assume "Scientific Magazine" is the name of a journal. If not, then you don't need capital letters.

I won't question the veracity of your sentences (hey, make it all up if you want), but the paragraph structure made it difficult to understand your point. Aim for clear phrases first and foremost.

The second effect can be seen upon a new relationship. The people who being left by one's boyfriend Someone who has been jilted can feel lonely because her boyfriend their ex-partner i.e her someone's - dare we say their right hand - has given them up her. For example, a person may break up with their partner when they start to find desire a new relationship. Thus, the "jilter" can get over this recover more easily than the "jiltee".

I really wasn't sure of your point here. Whose perspective are you speaking from?  There was some confusion with phrasal verbs, which made it even more difficult to understand.

Another note: avoid using "be" too often, as it is a very static verb. Use a more active verb if you can.

The final effect is a feeling of worthlessness. The people A person is a very valuable soul but they may descend fallinto sadness when they can feeling worthless. For instance To give a real-life example, my cousin broke up with her boyfriend while she was very sad because she thought of her ex-boyfriend was sadness by yourself. (I don't understand the end of this sentence.)

Taking these points into consideration, there are three effects from bring left by on's boyfriend upon people experiencing the end of a relationship, in terms of poorer performance at school, a new relationship and a feeling worthless. I obviously claim say that the people should not worry themselves about everything because the life is very short.

There's a misunderstanding of how <em>a</em> and <em>the</em> function. I suggest revising this part of grammar.

Aside from the errors, I join with Nick in congratulating you on an ambitious piece of work. :) Keep at it!