I missed many years of prayers which I cannot calculate and many years of fast. Rarely I do, but only when I felt. But now after learning more about my duties, really I am scared. I have lost all my life doing nothing. But now I am really particular of my prayers and fasts and am sincerely repenting. But what about years of missed prayers and fasts? What should I do? Also give me a beautiful dua to stick on my tongue for Allah to accept my Tawbah.
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Sister in Islam,
At the outset, we would like to commend you on the steps you have taken to turn away from your past life of disobedience. You should consider it a great bounty that you were granted the tawfīq (opportunity) to repent and connect yourself to the Deen of Allah Ta‘ālā. Many unfortunate people do not make tawbah (repentance) until it is too late. Remain steadfast, and make du‘ā to Allah to keep you firm on His Deen.
Allah Ta‘ālā is All-Forgiving and Compassionate. When a person turns back to Him with sincere tawbah, He will most assuredly forgive. If you have made sincere tawbah, be sure of forgiveness. Allah Ta‘ālā says:
قُلْ يَا عِبَادِيَ الَّذِينَ أَسْرَفُوا عَلَى أَنفُسِهِمْ لَا تَقْنَطُوا مِن رَّحْمَةِ اللَّهِ إِنَّ اللَّهَ يَغْفِرُ الذُّنُوبَ جَمِيعًا إِنَّهُ هُوَ الْغَفُورُ الرَّحِيم
“Say: O my slaves who have transgressed against their souls! Do not despair of the Mercy of Allah! Verily, Allah forgives all sins. Verily, He is All-Forgiving, Merciful.” (39:53)
And Rasūlullāh (sallAllāhu ‘alayhi wasallam) said:
التائب من الذنب كمن لا ذنب له
“The one who repents from sin is like the one who has no sin.” 
As far as your missed prayers and fasts are concerned, you must make up the Fard (compulsory) prayers, Witr prayers and the compulsory fasts of Ramadan which you missed since the age of maturity. If you are not able to calculate the precise number of missed prayers and fasts, make an overestimate until you are convinced that you have missed no more than that number.  Once you have done so, begin making up the prayers and fasts. For each prayer, make the intention of the first missed prayer of that particular Salāh that is due in your responsibility. For example, for each Zuhr prayer that you make up, make the intention of the first Zuhr that you missed that yet remains in your responsibility.  Do this for all the prayers until you have completed the number that you arrived at in your estimate. Do the same for the missed fasts.
One method you can use to help you complete this obligation is to offer each make-up (qadā’) prayer with the present prayer; for example, offer one missed Fajr immediately following the present Fajr, one missed Zuhr immediately following the present Zuhr, and so on. Alternatively, you can spare a portion of each day to make up the missed prayers of one whole day. You may also select certain days of the week to complete your missed fasts. Keep strict count of how many prayers and fasts you have made up until the number in your estimate is complete.
The following is a beautiful du‘ā taught to us by Rasūlullāh (sallAllāhu ‘alayhi wasallam) for seeking forgiveness:
أَسْتَغْفِرُ اللَّهَ الَّذِي لَا إِلَهَ إِلَّا هُوَ الْحَيّ الْقَيُّوم وَأَتُوبُ إِلَيْهِ
Astaghfirullah alladhee laa ilaaha illaa huwal hayyul qayyoom wa atoobu ilayh
“I seek forgiveness of Allah, the One besides Whom there is no deity, the Ever-Living, the All-Sustaining, and I turn (in repentance) to Him.” 
السنن لابن ماجه، دار الرسالة العالمية، ٥:٣٢٠
من لا يدري كمية الفوائت يعمل بأكبر رأيه فإن لم يكن له رأي يقض حتى يتيقن أنه لم يبق عليه شيء
حاشية الطحطاوي على مراقي الفلاح، دار الكتب العلمية، ٤٤٧
فتاوى محمودية، ٧:٣٦٤
فإذا كثرت الفوائت يحتاج لتعيين كل صلاة يقضيها…فإذا أراد تسهيل الأمر عليه نوى أول ظهر عليه أدرك وقته ولم يصله
مراقي الفلاح مع حاشية الدر، ٤٤٦
فتاوى محمودية، ٧:٣٦٥
سنن أبي داود، مؤسسة الريان، ٢:٢٩٥
And Allah Ta‘ālā Knows Best
Checked and Approved by,
Mufti Ebrahim Desai.