Go on some short courses in computer applications like Adobe Photoshop, etc, and Photography.
Send for the Prospectuses of local art/technical colleges to see what part- time courses in these and related graphics disciplines are on offer, and how they can be funded.
In the meantime , stop using the unfortunate term ' sketch- art', which gives people the impression you're just playing at it, and start thinking of yourself as a potential illustrator.
You can -
Keep sketchbooks / notebooks to collect visual/ written information related to graphics (from practising typefaces / Logos/signs, etc to costing and estimating)
Set some bigger tasks for yourself - eg a topic like ' Holidays' - design everything from the travel posters to the luggage labels and room service menus
Collect a cuttings scrapbook - fill it with samples of lettering, illustrations, labels, tickets , comic art etc that has interested you - all good graphic designers are eclectic in this way, keep files of them online too
Study the recent and distant history of ' commercial' art - lots of material available on vintage poster art, magazine illustration and so on
Eventually, you'll have enough skills and knowledge to decide what the next step is - I chose teaching , and loved teaching graphics, because even the most disaffected kids enjoyed it .
Good luck , hope this helps
Art/ design teacher for well over 30 years