Scopes and springers....
There's a lot of ground to cover here, especially since you quickly went from shooting fine to having trouble.
The first thing to do is check all 5 major screws on your rifle - the 3 that hold the stock to the rifle, and the 2 (possibly 1 with a nut opposite it) that controls the gap between the action fork and the pivoting block that the barrel screws into. After you check screws, try to wiggle the barrel in its fork - if it has a bit of wiggle you may need a slender washer as a shim. I did that to a box store Crosman a couple of years ago and it went from 2" or more at 10 yards to around 3/8". 10 cent shim...
If they are all good and snug, and you still don't shoot well, a scope is probably the way to go.
Springers eat scopes. Period. Big rifles eat them faster. Modest rifles and good scopes can go a long ways though.
I recommend a good scope with an adjustable objective (very important IMO) with a no questions lifetime warranty.
There are multiple options, the most popular seems to be the Hawke Vantage with AO. Many sizes and reticle, with 3-9x40 & mil dot likely the most common. $100 or so should be about right these days.
The warranty is only for the original owner (keep your receipt!) and the scope is recommended to rifles in the 12 fpe or less category. Your rifle should be OK.
Bushnell makes certain models (Trophy and up I think) that are warranteed for springers, and there are definitely less expensive options like Hammers, UTG, etc. There are also high end scopes like Leupold, etc but that's a lot of money to shoot at 25 yards or so...!
One other thing - springers really can't be held firmly, the way someone would naturally hold a rimfire for example. If you tried to hold it tighter when you first starting having trouble, that could be a factor too.
