Like another user has said, I'd suggest a skincare routine revamp.
What's your current routine like?
Dealing with acne and post-inflammatory hyperpigmentation (PIH; this is the red/purple marks a blemish leaves behind) is a multi-step process.
If you haven't already got a sunscreen that is broad-spectrum, photostable and an SPF of 30 or above you should include that first and foremost as the sun can make the PIH worse. Both chemical and physical sunscreens are OK for this though some people might be sensitive to chemical sunscreens and conversely, physical sunscreens are less cosmetically elegant because they leave a white cast on the skin. No, the SPF in your foundation/moisturiser/etc. isn't enough, and to be honest, you should really reapply sunscreen throughout the day. Also, to properly remove sun screen you need an oil-based cleanser.
When you have found a sunscreen that works for you, the next step that you could try is to add in a CHEMICAL exfoliant, not physical. Physical exfoliants (looking at you St. Ives Apricot scrub!) are actually incredibly damaging to the skin and causes micro-tears in the skin which can leave you prone to more breakouts. Chemical exfoliants include things like AHAs (glycolic, mendalic, lactic, etc.) and BHAs (salicylic). BHAs are good at removing dirt from clogged pores and AHAs are good at brightening the skin. This is because BHAs are oil-soluble but AHAs are only water-soluble. If you do decide to use both in your routine I would advise that you only use them at night and use the BHA first. Additionally, the other reason why I suggest you first look into getting a broad-spectrum, photostable sunscreen is because AHAs can make your skin more photosensitive.
Other treatments (apart from other things also stated in this thread, e.g. retin-A) that could help with your acne could be benzoyl peroxide, but I don't have any experience with this myself.
Other things to help with the PIH include things like L-ascorbic acid (aka VitC) and to lesser/varying extents its derivatives. Most people opt for L-ascorbic acid's derivatives due to it being so unstable when it's in water (which it usually is, although it can be more stable when it's in a formula with vitamin E and ferulic acid). It also can't be used at the same time as niacinamide as they cancel each other out. If you want to use both, I'd say use the LAA in the morning and the niacinamide at night as the LAA can boost your sunscreens effectiveness. Not sure if there's any contradictions with its derivatives though as most of them have much less research. I know The Ordinary by Deciem has an anhydrous version of L-AA and it's therefore supposedly more stable but a lot of reviews say it's pretty gritty and therefore not very good to use under makeup etc. Azelaic acid also has some research to support it's helpfulness in rosacea/skin redness but there's not a whole lot.
I would recommend looking at The Ordinary products though, they are pretty darn good for the price and they tend to be well formulated for the most part.
Other than that, perhaps head over to SkincareAddictions subreddit
- loads of information over there, they all have way more knowledge than me. There's also a UK version
of it too!
Also, it might also be worth talking to your GP for a dermatologist referral and drinking more water too. Diet plays a big role in skin condition, as does stress.
PS. Going without makeup might help too, if you're able to (don't forget to wash it off at night! Oil-based cleansers are amazing for this! The two-step cleanse method is really good. SkincareAddiction has a lot about it. )