The standard error of the regression line at point $X$ (i.e. $s_{\widehat{Y}_{X}}$) is hand calculated (Yech!) using:
$s_{\widehat{Y}_{X}} = s_{Y|X}\sqrt{\frac{1}{n}+\frac{\left(X-\overline{X}\right)^{2}}{\sum_{i=1}^{n}{\left(X_{i}-\overline{X}\right)^{2}}}}$,
where the standard error of the estimate (i.e. $s_{Y|X}$) is hand calculated (Double yech!) using:
$s_{Y|X} = \sqrt{\frac{\sum_{i=1}^{n}{\left(Y_{i}-\widehat{Y}\right)^{2}}}{n-2}}$.
The confidence band about the regression line is then obtained as $\widehat{Y} \pm t_{\nu=n-2, \alpha/2}s_{\widehat{Y}}$.
Bear in mind that the confidence band about the regression line is not the same beast as the prediction band about the regression line (there is more uncertainty in predicting $Y$ given a value of $X$ than in estimating the regression line). And, as you are struggling to understand, the confidence intervals about the intercept and slope are yet other quantities.
Further, you do not understand confidence intervals: "if in 95% of the cases my estimates are within the confidence interval, these seem like a possible outcome?" Confidence intervals do not 'contain 95% of the estimates,' rather for each separate sample (produced by the same study design), 95% of the (separately calculated for each sample) 95% confidence intervals would contain the 'true population parameter' (i.e. the true slope, the true intercept, etc.) that $\widehat{\beta}$ and $\widehat{\alpha}$ are estimating.
Best Answer
The following R code is copied verbatim from Appendix C, page C-20 of U.S. EPA's Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities; Unified Guidance; March 2009 (EPA 530/R-09-007; https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P10055GQ.TXT). The code was written by Kirk M. Cameron, Ph.D. of MacStat Consulting, Ltd.