When the body is stiff and inflexible from years of improper training and leading a sedentary lifestyle, it simply takes time for the fascia, musculature, and nervous system to adapt in order to gain new ranges of motion. Many adults try a stretch for the first time, notice how hard it is, and just decide that it isn’t for them. Instead, try to reframe the situation, and make a commitment to gradually chipping away at the work that needs to be done in order to get yourself to where you want to be.
The problem is that most training is based off muscular fatigue. Muscles adapt — we’re going to replace all your muscle tissue in about 90 days. It’s going to take 200-210 days to replace all of your connective tissue. So if training is only based on what muscle tissue does, the body gets further and further behind on what the connective tissue can handle, which means if you’re one of the people that only likes to push hard in training, you’re just an accident waiting to happen. Use your progressions and mobility as a guide to build a stronger body, trying to rush the progress will result in time consuming injuries.