The first thing you'll want to do is cut out the silicones, which most Tigi products are full of - a silicone is anything ending in 'cone or 'xane, except for the preservatives towards the end of the ingredient list. If you're using that and a sulfate free shampoo, you aren't ridding yourself of the silicones and they will build up on your hair and block out any moisture you're feeding it. Start with a clarifying shampoo (without silicones, one of the best and cheapest is any of the suave naturals) and toss the Tigi stuff and anything with cones.
Find a good rinse out conditioner that is moisturizing, such as GVP conditioning balm (from Sally's) or anything richer without silicones - are you still living in Belgium? I'm not sure what the product market is over there. If you're in the US now, you should be able to find most of the products talked about around here.
Feed it some moisture without silicones and you should see a pretty big improvement just from that. Next, you'll want to determine the rest of your hair properties, the most important one being texture. Porosity plays a big role in hair health as well. Highly porous hair is generally a result of chemical or heat damage, but since you don't straighten, it's possible you have naturally highly porous hair. There is a great link I'll post for you in helping to determine texture. Once you figure that out, you'll have more direction where to go for products. Fine hair usually prefers protein in products, especially if it's porous, and coarse hair not so much. Might help if you post a pic too!!
Here's a link for determining the basics, and hair properties: Live Curly Live Free - Curly Hair Basics
I couldn't find the link for the protein recipe, but it's easy to do at home, customizeable, and cheap. Jas76 posted a walk through in one of her threads, I will link it, hope she doesn't mind... Lazy gelatin PT leads to great hair!!!
Another good conditioner that is moisturizing and has protein is ION Effective Care. I love that stuff and can use it as a RO.
Once you nail down properties and see how your hair responds to co-washing and/or a sulfate free shampoo, you might discover you need to clarify or use a sulfate poo (which is common among wavies with thin hair or weak waves) and you can at that point work one back in, in moderation. I use T Gel for scalp issues and it has SLES instead of SLS, which is really harsh, and the SLES doesn't bother me as long as I counteract it with a good RO. Another good cleanser that isn't as harsh as SLS is cocomidopropyl betaine. But right now it sounds like you need to focus on moisture first and foremost.
Figure out your hair properties first, and post back here with them and if I can come up with any more advice based on that, I will certainly do so! HTH!