Even the first textbooks that start introducing abstract math and developing mathematical maturity in beginning college students often (usually?) have problem sets without solutions, and I'm defending that. I think it's better than providing solutions, even for people working alone.
Of course it isn't ideal to work alone; it's helpful to have other people to point out errors in your proofs and to present their own proofs to you. I just don't think providing solutions helps at all in that way. What gives you the best approximation of that experience is spending a long time working on problems. Sometimes you'll realize you've proved something false and have to debug your own proof, so you'll be collaborating with yourself. Solutions will only make it harder for you to force yourself to have those experiences.