the verb is demek= to say

Beni unutsun demişsin
appearently (or: it was said that) you've told her/him to forget me

Bunu sen diyemeşsin
"diyemeşsin" isn't a correct word, maybe it should be "diyemezsin"- you can't say that or "diyememiştin"- you couldn't have said that

that's what I think, an expert's help will be appreciated

Everything is beautiful with you! I have only one wish, be happy that's enough but (the last word doesn't make much sense, as it is it means "we are a lie" but maybe it should be "yalnız"- alone)

Thank you for the mail you've sent. I don't know how you've given me a chance. But as you're thinking of corresponding with Semra I can't interfere with that. As I have said before I don't know you. But maybe Semra knows you. Since you know us so well you must be very near to us. Anyway take care, may everything be as you want. I'll still wait for your mails. Have a good day.

I think the last line should be:
ve sen aşkın yanlız şanslı ve güçlü olanlar için olduğunu düşÃ¼nüyorsun

do you mean that we can't stay together my only one, why does your friend want to stay, is your friend going to come with you or s/he's here?

You are as valuable as the rain which will rain in the desert, you are as beautiful as the roses that open on their branches, you are a pearl which shouldn't be lost and if lost can't be found, my only one I kiss you.

"Sen benden gittin gideli" means: as soon as you went away from me

second part:

By the end of spring I have to take some decisions concerning my life and for that I'm in a struggling situation, but because of the complicated and wrong going situations I can't think straight and I'm afraid that the time I've given myself would take longer, because I haven't left much strength...
If you came to Istanbul we would have had a beautiful vacation, that's true... we would have spent wonderful time, just like in Spain, but at the end we would both remain with pain, I don't find that right... Because of the respect I have for myself and for you I have to refuse some things... please understand that... I like you, but I'm afraid of loving, you are right... because whenever I love I suffer pain... anyway...
what I've written won't satisfy you, but that's all from me for now... only time XXX only time... and believe that it won't be long...
with my love...

translation:
I've read the mail you sent me and you are right about many subjects you think about me. But as I've told you before I need time. What I mean by time aren't really decisions I have to make that are concerned with you. I've always said that I need time to make decisions about myself, and while taking those decisions I don't want our relationship to be worn-out (ruined), that's why I'm continuously giving you an answer of refusal, I hope you understand that. During this time the only thing I wanted from you was not to insist and not to talk about that. But in our every conversation you asked me about us. As for me I didn't have an answer to give to you and I still don't...
(to be continued)