A value proposition is the "thing" that you do, how you do it and "WHY" you do it that has value to your customer and you happen to use in your proposal.
See Simon Sinek - - The Why on YouTube. This will help a LOT !
There are some things which I would need to 'ask' if we were on a call together...
How long have you been trading?
Do you have existing clients?
If the answer is "not very long". And, "Just one or two clients", then the simple answer is just trade... trade, trade and trade some more. Win as much business as you can. Work with as many clients as you can. And have one core fundamental mission at all times:
To ascertain why those clients chose you.
To ascertain why they came back and used you again.
To break down everything you do - from your smile, to your proposals, to your deliverables... and de-construct it to the point where you can genuinely say...
"I am unique because..."
"I can see how I could be unique by..."
There is a secret to business (especially service businesses). It's all about the people and the methodology. If you are trading... and people are buying... you are doing something special. Once you understand what that is, then that is the key bit of value that the clients actually find valuable above and beyond the rest.
If you are a service business, then the meticulous replication of the service is crucial. See Mcdonalds. See Apple Stores. Impeccable.
By trading, you get the opportunity to have deeper conversations with your clients/customers. It will be the moments of un-structured conversation where they say something like, "Oh I wish XYZ"... or "Why can't we just do it 'this' way instead". Those types of conversations are where you should have a lightbulb moment. That is the time (because you are looking for it) that you will be able to adjust, pivot, augment and morph your service into something different from your competition. It will come from your customers. Why? Because who are you to say what has value to them... They need to tell you what has value to them... and it is your job to extract it from them - - because they rarely know properly themselves.
If you are looking for it - - you will find it. Check out http://mjskok.com/ or watch the Harvard iLabs material on Value Propositions.
The best way to find our elusive difference ... is to be in the business, trading hard... and looking and honing and toning and constantly asking your customers (or anyone) how you could do it better, differently, faster etc that has value to them... and in a way that you can deliver.
Then you just need to make sure you up your skills and build your business around that value so that competitors can't easily copy it.
Good luck !