First of all can I say I am sorry to hear of the loss of your son, having never experienced it I can't say I understand, but my heart goes out to you and your family.
Second well done with the last 6 months, it has been a difficult time to start window cleaning but you do seem to have done well to have the customers you do have in that time.
Now to your situation. I would say there may be a problem if you have lost as many as 25 in 1 month. Your regular so its not that. I will presume you do a good job! And I will presume you are not a flake!! Why would I, I don't know you well enough.
It could just be that as a Newbie you have uneducated customers, or customers that others have dropped because of being unreliable. It takes time to sift through these type.
As for giving customers a "terms of business", my personal view is that it can come across as a big stick to beat the customer with. I tried one myself some time back and my relationship with the customer changed for the worse, I didn't secure better customers as a result of it (in fact it put some off) and it created a lot of paperwork, as to be effective you need people to sign something and keep a record of it.
If they break the agreement ask yourself what would you do. If they said "Not today" and you drop them for good you have lost not only this months money but the next few as well.
If they fail to unlock the back gate and you charge them full price you risk isolating them. Remember their window cleaner dose not figure high on their table of important things.
Plus, you have to keep to YOUR terms of business. "We offer a monthly service" great. But what happens when you take 2 weeks off in the summer, they then get a late clean or you have to do a months work in 2 1/2 weeks. You then are not keeping your side of the bargain.
My advice would be to find out specifically why you have lost 25 in a month, even go and ask them. Ask them to be honest, be polite and tell them that they are welcome to call you whenever they wish to resume. Then work out how to keep customers. I have customers who will not have anyone else clean their windows, even if they charged half the price. Hitting them with a page of T & C isn't really the way forward. There is an excellent example in a book I have, I will dig it out. it's about behavioural economics.
There are 2 ways to deal with customers. Secular and Familiar. With Secular you would have T&C. You dont get to close, Surname not first name terms, if you say you call every first Tuesday of the month then you do. If you are away then your staff call. This can work well, but you would not expect a cup of tea or a chat. You give an ultra proffesional service, they recieve the same.
With familiar things are the opposite, you are on first name terms, you have a chat, you almost become a friend. You make yourself unreplacable by giving agreat service and by being so nice that they don't have the heart to stop you calling. They may need you to skip a visit every now and then, but hey your their friend you will understand. You reply to this with "No problem" then ask them to think of a friend they can refer to you to to help your business grow. 9 times out of 10 they will give you a reference and not skip a month in future as they don't want to be "replaced".
The benefit of Familiar is that when you do go away for 2 weeks or when you do have a bad day and make a bad job, they are more likely not to complain.
We all run our businesses slightly differently. Whatever approch you take, you have to be consistant. You cant mix secular with familiar very well.
I hope things pick up for you. Keep at it!!