Hi Sandra,
Since your project isn't until autumn. Find a "professional vocal coach" with references to work with you. A pro vocal coach NOT a "voice teacher". A real pro vocal coach will tailor your sessions to fix problems and refine your own singing style as opposed to just learning breathing, scales, etc. The cost will be worth it as the real pro vocal coach will help you be the best you personally can be in the least amount of time. You'll do the work and approach singing with the same professional mindset as you do with perfecting your guitar chops - which I believe from what you said is what you want to do.
Once you add a vocal to a song - whether the singer is you or somebody else - that vocal now carries that song. The majority of people listening will focus on the vocal.
I disagree with some here who say your vocal songs can be somehow expendable or lesser just because you are foremost an instrumentalist. This is a singles oriented world now. There are no designated album cuts any more. Once you step up to the mic people have expectations. You're going to take heat for doing it from all the critics regardless. You don't have to be the reigning Ms Golden pipes but you can be the best singer Sandra can be with your own confident style.
Ask around and find yourself a real pro vocal coach who's working or has worked with other budding artists who have label deals and go from there.

All the best,
Josie