If i can't prove adultery by wife then can i get divorce...

Of course yes. Though adultery is not proved, it reveals her character. But mainly fight your case on the ground of cruelty as you do not have any direct evidence that she was in physical relation with someone......

Adultery is not a ground for divorce under Indian law. Cruelty is. If you can submit proof of adultery you can only present it as cruelty done to you. In fact, the evidence that you already have - letters are good proof for cruelty. You can present a pretty good case of mental cruelty.

Make sure you provide all circumstances in detail. I think you have a pretty good ground for divorce.

Good luck

Disclaimer : I am not a lawyer.

without direct proof only with some evidences which cannot prove adultery 

with those proves you can give direct attack on cruelty and to some extent mental torture also and also try before going for divorce act if you can find any more proof

Thanks for your thoughts and Advice. Reason I want to prove adultery, I want to get rid of Maintenance part.

There was article in TOI, Adulterous wife can't get Maintenance.