Forgetting how to structure a sentence.

Hi,
For a year now, I am finding it difficult to speak and pronounce words correctly. Also, I at times, find it hard to get the grammar right. For 14 years, I have been able to speak fluent English as I was born and raised in England. I am not dyslexic because I took the test and there are only a few signs of dyslexia. This occurs when I speak and write. A year ago, I stopped speaking to people. This was a bad decision as I lost my confidence and my social life. I lost friends and my self esteem. My English was fine till last year around April. I believe Tamil, my second language, is the reason for this. This is because I speak Tamil all the time at home, decreasing my knowledge in English. What I need to do is to surround myself with English but I don't have time and my GCSEs are coming up.
In addition, I find it hard to pronounce complicated and not so complicated words and i find it hard to say words like Erica, decision, wall, ill, marketable and etc.
Also, it takes me time to structure a sentence that makes sense. In addition, i get the grammar missed up. For example, do, does, have, had, is, are, ...s, etc.
For month, i have been coming up with theories and solutions but i haven't uncovered the real problem. It has been driving me mad for months and i am slowly losing my confidence and self image. I am worried but I don't want to give up because there is always a way around thing. This is why i am asking you guys to seek any other sufferers. My GCSE's are appearing next year and so it is a must to get this solved. I feel more like a foreigner who can't speak English at all and the intimidating part is that i was born in this country and i have British english friends. It is very embarrassing when you make a mistake in front of them and when you try to pronounce words properly.
I hope you can help me.

D