This is a simple problem to fix.
Flow the PTC beginner program on this site. This program will last you easily 12 months.
Next. eat more. I don't agree with eating everything in sight and turning yourself into a fat fuck though.
Aim for .5kg weight gain a week initially. If you don't gain this, eat more. If you gain over this, eat a bit less. Should be no problem for you for a while but if adding too much fat slow weight gain back to a kg a month.
Do this for at least 3 years then go on a nice slow cut.