Usually, some people will post some or almost all of the solutions online if the textbook is really famous, for instance, baby rudin.

Although rarely, some authors do provide solutions, like Knuth's books, Stephen Abbott's Understanding Anaylsis, etc.

For immediate feedback, maybe you can checkout [0] to learn some formal proof by doing interactive proving.

[0] http://wwwf.imperial.ac.uk/~buzzard/xena/natural_number_game...

BTW, you can always ask questions on https://math.stackexchange.com