Proposing to an old lady at the bank

the other day i was at the bank for some paperwork and unfortunately i can't read Japanese, so an old lady saw me struggling between the documents and my e-dictionary trying to figure out what is what, she approached me and gently offered to help me filling the application. after she did i really wanted to show my gratitude to her for helping me, so i squeezed my mind trying to remember a very polite way to thank her in Japanese language,........ there it is i found it "hontou ni kekkonshteimasu" is said to her .... she smiled to me and left ...... I headed to the exit door filled with happiness because i managed to really express my gratitude for the lady. and suddenly i just felt the hit of a big train in my head because i just realized that in stead of thanking the lady i proposed for a marriage, because i did not say the correct sentence for the occasion which would be "hontou ni kanshashiteimasu". but i said " kekkonshteimasu" which means i am in a state of being married or some times can be mistakenly be understood as i want to marry you. now here is how to say i want to marry you "anata to kekkonshitai" Greetings