This is a thank you letter to a company. Please check it and let me know what you think. If I need to change something or its just fine. Check for any errors.

Thank you very much for taking the time to meet with me on April 14 regarding the position of Administrative Assistant.

Our conversation increased my already-significant interest in the position. I enjoyed learning more about ABC Company and was especially impressed by the plans you outlined to make ABC Company a better place. I am confident that I can make an immediate and significant contribution to the company.

Again, thank you for your time and interest. I'm looking forward to help make your company succeed.

I'm looking forward to helping ...

Everything else sounds fine.

Not sure that you need that dash in "already-significant"

In the end, change "help" to "helping."
Better yet, you could say "I'm looking forward to contributing to the success of your company."

Dear [Company Name],

I am writing this letter to express my gratitude for the opportunity to meet with you on April 14th to discuss the Administrative Assistant position at ABC Company. I would also like to seek your feedback on the letter and make any necessary changes to ensure its effectiveness.

Thank you for taking the time to meet with me and allowing me to learn more about ABC Company. Our conversation only further solidified my keen interest in the position. Hearing about your plans and vision for the company left a lasting impression on me. I am truly impressed by the proactive measures you have outlined to make ABC Company an even better workplace.

Considering my professional background and skills, I am confident that I can make an immediate and significant contribution to the growth and success of ABC Company. I am eager to utilize my expertise and dedication to help drive the company towards achieving its goals.

Once again, thank you for your time and interest in meeting with me. I greatly appreciate the opportunity to be considered for the Administrative Assistant position at ABC Company. I look forward to the chance to contribute to the company's continued success and growth.

Please let me know if there are any changes or adjustments that you would recommend to ensure the letter accurately reflects my appreciation and interest in the position.

Thank you once again for your consideration.

Sincerely,
[Your Name]